adding -vvvv debug options seems to have explained the issue - as mentioned before that large transfer of many small files must have caused, what seems to be "transfers excceeded" limit in google drive api - truncated log below:
~ 3m 35s ❯ rclone mount gdrive-crypt: /home/<USER>/gdrive-rclone-crypt --umask 077 --rc --rc-addr :<PORT> --rc-user <USER> --rc-pass <PASSWORD> --cache-dir /home/<USER>/.cache/rclone/gdrive-encrypted --vfs-cache-mode full --vfs-cache-poll-int
erval 5m --vfs-cache-max-age 8760h --vfs-cache-max-size 10G --attr-timeout 8700h --dir-cache-time 8760h --poll-interval 10s --multi-thread-streams 0 -vvvv
2024/10/06 13:29:26 DEBUG : rclone: Version "v1.68.0" starting with parameters ["rclone" "mount" "gdrive-crypt:" "/home/<USER>/gdrive-rclone-crypt" "--umask" "077" "--rc" "--rc-addr" ":<PORT>" "--rc-user" "<USER>" "--rc-pass" "<PASSWORD>" "-
-cache-dir" "/home/<USER>/.cache/rclone/gdrive-encrypted" "--vfs-cache-mode" "full" "--vfs-cache-poll-interval" "5m" "--vfs-cache-max-age" "8760h" "--vfs-cache-max-size" "10G" "--attr-timeout" "8700h" "--dir-cache-time" "8760h" "--poll-inter
val" "10s" "--multi-thread-streams" "0" "-vvvv"]
2024/10/06 13:29:26 INFO : Using --user <USER> --pass XXXX as authenticated user
2024/10/06 13:29:26 NOTICE: Serving remote control on http://[::]:<PORT>/
2024/10/06 13:29:26 DEBUG : Creating backend with remote "gdrive-crypt:"
2024/10/06 13:29:26 DEBUG : Using config file from "/home/<USER>/.config/rclone/rclone.conf"
2024/10/06 13:29:26 DEBUG : Creating backend with remote "gdrive:crypt"
2024/10/06 13:29:26 DEBUG : vfs cache: root is "/home/<USER>/.cache/rclone/gdrive-encrypted"
2024/10/06 13:29:26 DEBUG : vfs cache: data root is "/home/<USER>/.cache/rclone/gdrive-encrypted/vfs/gdrive-crypt"
2024/10/06 13:29:26 DEBUG : vfs cache: metadata root is "/home/<USER>/.cache/rclone/gdrive-encrypted/vfsMeta/gdrive-crypt"
2024/10/06 13:29:26 DEBUG : Creating backend with remote ":local,encoding='Slash,Dot':/home/<USER>/.cache/rclone/gdrive-encrypted/vfs/gdrive-crypt/"
2024/10/06 13:29:26 DEBUG : :local: detected overridden config - adding "{bxYPm}" suffix to name
2024/10/06 13:29:26 DEBUG : fs cache: renaming cache item ":local,encoding='Slash,Dot':/home/<USER>/.cache/rclone/gdrive-encrypted/vfs/gdrive-crypt/" to be canonical ":local{bxYPm}:/home/<USER>/.cache/rclone/gdrive-encrypted/vfs/gdrive-crypt"
2024/10/06 13:29:26 DEBUG : Creating backend with remote ":local,encoding='Slash,Dot':/home/<USER>/.cache/rclone/gdrive-encrypted/vfsMeta/gdrive-crypt/"
2024/10/06 13:29:26 DEBUG : :local: detected overridden config - adding "{bxYPm}" suffix to name
2024/10/06 13:29:26 DEBUG : fs cache: renaming cache item ":local,encoding='Slash,Dot':/home/<USER>/.cache/rclone/gdrive-encrypted/vfsMeta/gdrive-crypt/" to be canonical ":local{bxYPm}:/home/<USER>/.cache/rclone/gdrive-encrypted/vfsMeta/gdriv
e-crypt"
2024/10/06 13:29:28 DEBUG : mailing list archive 2/1432252252.MgzgVltJxlylX_0551.tachyon,S=7680:2,S: vfs cache: checking remote fingerprint "7680,2016-08-21 00:43:29 +0000 UTC" against cached fingerprint ""
2024/10/06 13:29:28 DEBUG : mailing list archive 2/1432252252.MgzgVltJxlylX_0551.tachyon,S=7680:2,S: vfs cache: truncate to size=7680 (not needed as size correct)
2024/10/06 13:29:28 DEBUG : vfs cache: looking for range={Pos:0 Size:7680} in [{Pos:0 Size:7680}] - present true
2024/10/06 13:29:28 DEBUG : mailing list archive 2/1432252252.MgzgVltJxlylX_0551.tachyon,S=7680:2,S: vfs cache: setting modification time to 2016-08-21 01:43:29 +0100 BST
2024/10/06 13:29:28 INFO : mailing list archive 2/1432252252.MgzgVltJxlylX_0551.tachyon,S=7680:2,S: vfs cache: queuing for upload in 5s
2024/10/06 13:29:28 DEBUG : mailing list archive 2: Added virtual directory entry vAddFile: "1432252252.MgzgVltJxlylX_0551.tachyon,S=7680:2,S"
2024/10/06 13:29:28 DEBUG : mailing list archive 2/1432252252.MhabBzjWs4sKX_0241.tachyon,S=5953:2,S: vfs cache: checking remote fingerprint "5953,2016-08-21 00:44:04 +0000 UTC" against cached fingerprint ""
2024/10/06 13:29:28 DEBUG : mailing list archive 2/1432252252.MhabBzjWs4sKX_0241.tachyon,S=5953:2,S: vfs cache: truncate to size=5953 (not needed as size correct)
2024/10/06 13:29:28 DEBUG : vfs cache: looking for range={Pos:0 Size:5953} in [{Pos:0 Size:5953}] - present true
2024/10/06 13:29:28 DEBUG : mailing list archive 2/1432252252.MhabBzjWs4sKX_0241.tachyon,S=5953:2,S: vfs cache: setting modification time to 2016-08-21 01:44:04 +0100 BST
2024/10/06 13:29:28 INFO : mailing list archive 2/1432252252.MhabBzjWs4sKX_0241.tachyon,S=5953:2,S: vfs cache: queuing for upload in 5s
2024/10/06 13:29:28 DEBUG : mailing list archive 2: Added virtual directory entry vAddFile: "1432252252.MhabBzjWs4sKX_0241.tachyon,S=5953:2,S"
2024/10/06 13:29:28 DEBUG : mailing list archive 2/1432252252.Mhhhof5pu1W4h_0590.tachyon,S=1298:2,S: vfs cache: truncate to size=1298 (not needed as size correct)
2024/10/06 13:29:28 DEBUG : mailing list archive 2/1432252252.Mhhhof5pu1W4h_0590.tachyon,S=1298:2,S: vfs cache: setting modification time to 2016-08-21 01:43:39 +0100 BST
2024/10/06 13:29:28 INFO : mailing list archive 2/1432252252.Mhhhof5pu1W4h_0590.tachyon,S=1298:2,S: vfs cache: queuing for upload in 5s
2024/10/06 13:29:28 DEBUG : mailing list archive 2: Added virtual directory entry vAddFile: "1432252252.Mhhhof5pu1W4h_0590.tachyon,S=1298:2,S"
2024/10/06 13:29:29 DEBUG : mailing list archive 2/1432252252.MiK08UYiidir6_0786.tachyon,S=8087:2,S: vfs cache: truncate to size=8087 (not needed as size correct)
2024/10/06 13:29:29 DEBUG : mailing list archive 2/1432252252.MiK08UYiidir6_0786.tachyon,S=8087:2,S: vfs cache: setting modification time to 2016-08-21 01:44:10 +0100 BST
2024/10/06 13:29:29 INFO : mailing list archive 2/1432252252.MiK08UYiidir6_0786.tachyon,S=8087:2,S: vfs cache: queuing for upload in 5s
2024/10/06 13:29:29 DEBUG : mailing list archive 2: Added virtual directory entry vAddFile: "1432252252.MiK08UYiidir6_0786.tachyon,S=8087:2,S"
2024/10/06 13:29:29 DEBUG : mailing list archive 2/1432252252.MiNyV9R82Gkf9_0086.tachyon,S=1750:2,S: vfs cache: truncate to size=1750 (not needed as size correct)
2024/10/06 13:29:29 DEBUG : mailing list archive 2/1432252252.MiNyV9R82Gkf9_0086.tachyon,S=1750:2,S: vfs cache: setting modification time to 2016-08-21 01:47:28 +0100 BST
2024/10/06 13:29:29 INFO : mailing list archive 2/1432252252.MiNyV9R82Gkf9_0086.tachyon,S=1750:2,S: vfs cache: queuing for upload in 5s
2024/10/06 13:29:29 DEBUG : mailing list archive 2: Added virtual directory entry vAddFile: "1432252252.MiNyV9R82Gkf9_0086.tachyon,S=1750:2,S"
2024/10/06 13:29:29 DEBUG : mailing list archive 2/1432252252.Mie9jxfU3GAax_0326.tachyon,S=1956:2,S: vfs cache: truncate to size=1956 (not needed as size correct)
2024/10/06 13:29:29 DEBUG : mailing list archive 2/1432252252.Mie9jxfU3GAax_0326.tachyon,S=1956:2,S: vfs cache: setting modification time to 2016-08-21 01:44:53 +0100 BST
2024/10/06 13:29:29 INFO : mailing list archive 2/1432252252.Mie9jxfU3GAax_0326.tachyon,S=1956:2,S: vfs cache: queuing for upload in 5s
2024/10/06 13:29:29 DEBUG : mailing list archive 2: Added virtual directory entry vAddFile: "1432252252.Mie9jxfU3GAax_0326.tachyon,S=1956:2,S"
...
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MYxp7V1tzTE0V_0230.tachyon,S=5883:2,S: md5 = 6c408ae57aacb585b647442e68fbbb0d OK
2024/10/06 13:31:54 INFO : mailing list archive 2/1432252253.MYxp7V1tzTE0V_0230.tachyon,S=5883:2,S: Copied (new)
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MYxp7V1tzTE0V_0230.tachyon,S=5883:2,S: vfs cache: fingerprint now "5883,2016-08-21 00:49:17 +0000 UTC"
2024/10/06 13:31:54 INFO : mailing list archive 2/1432252253.MYxp7V1tzTE0V_0230.tachyon,S=5883:2,S: vfs cache: upload succeeded try #1
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MaDgkYJyd4nL2_0873.tachyon,S=1503:2,S: vfs cache: delaying writeback as --transfers exceeded
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MZrBtRTBRYBVB_0442.tachyon,S=2212:2,S: vfs cache: starting upload
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MzWRAGwvj2lDL_0292.tachyon,S=2560:2,S: vfs cache: truncate to size=2560 (not needed as size correct)
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MzWRAGwvj2lDL_0292.tachyon,S=2560:2,S: vfs cache: setting modification time to 2016-08-21 01:52:19 +0100 BST
2024/10/06 13:31:54 INFO : mailing list archive 2/1432252253.MzWRAGwvj2lDL_0292.tachyon,S=2560:2,S: vfs cache: queuing for upload in 5s
2024/10/06 13:31:54 DEBUG : mailing list archive 2: Added virtual directory entry vAddFile: "1432252253.MzWRAGwvj2lDL_0292.tachyon,S=2560:2,S"
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MaDgkYJyd4nL2_0873.tachyon,S=1503:2,S: vfs cache: delaying writeback as --transfers exceeded
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MzWRGlkehSXSj_1012.tachyon,S=2291:2,S: vfs cache: truncate to size=2291 (not needed as size correct)
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MzWRGlkehSXSj_1012.tachyon,S=2291:2,S: vfs cache: setting modification time to 2016-08-21 01:53:47 +0100 BST
2024/10/06 13:31:54 INFO : mailing list archive 2/1432252253.MzWRGlkehSXSj_1012.tachyon,S=2291:2,S: vfs cache: queuing for upload in 5s
2024/10/06 13:31:54 DEBUG : mailing list archive 2: Added virtual directory entry vAddFile: "1432252253.MzWRGlkehSXSj_1012.tachyon,S=2291:2,S"
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MaDgkYJyd4nL2_0873.tachyon,S=1503:2,S: vfs cache: delaying writeback as --transfers exceeded
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MZ4xrzKLqI139_0565.tachyon,S=1063:2,S: md5 = 689377d2126509af04c2ac3af9e67c08 OK
2024/10/06 13:31:54 INFO : mailing list archive 2/1432252253.MZ4xrzKLqI139_0565.tachyon,S=1063:2,S: Copied (new)
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MZ4xrzKLqI139_0565.tachyon,S=1063:2,S: vfs cache: fingerprint now "1063,2016-08-21 00:43:40 +0000 UTC"
2024/10/06 13:31:54 INFO : mailing list archive 2/1432252253.MZ4xrzKLqI139_0565.tachyon,S=1063:2,S: vfs cache: upload succeeded try #1
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MaITIdGtLrc14_0019.tachyon,S=1217:2,S: vfs cache: delaying writeback as --transfers exceeded
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MaDgkYJyd4nL2_0873.tachyon,S=1503:2,S: vfs cache: starting upload
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MzWVLFuEZrefB_0136.tachyon,S=3605:2,S: vfs cache: truncate to size=3605 (not needed as size correct)
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MzWVLFuEZrefB_0136.tachyon,S=3605:2,S: vfs cache: setting modification time to 2016-08-21 01:52:15 +0100 BST
2024/10/06 13:31:54 INFO : mailing list archive 2/1432252253.MzWVLFuEZrefB_0136.tachyon,S=3605:2,S: vfs cache: queuing for upload in 5s
2024/10/06 13:31:54 DEBUG : mailing list archive 2: Added virtual directory entry vAddFile: "1432252253.MzWVLFuEZrefB_0136.tachyon,S=3605:2,S"
2024/10/06 13:31:54 DEBUG : mailing list archive 2/1432252253.MaITIdGtLrc14_0019.tachyon,S=1217:2,S: vfs cache: delaying writeback as --transfers exceeded
I'm hoping that this will sort itself on its own after a while...