I got the same errors today. Uploading plain text files, not even open, using a simple sync on Windows. Never seen such errors before.
Though I did notice some other strange errors a few days ago, not sure if it is related:
error computing hash: failed to open object to read nonce: error 404: no.jotta.backup.errors.PhysicalFileNotFoundException: Unable to read size on remote dfs (404); location=FileLocation{SEAWEED|sw-0.....} (Not Found)
That was with cryptcheck. Found two mentions of similar errors, one with positive conclusion:
Just got a reply from Jottacloud and they said they've had some issues regarding this, but it's resolved now.
(https://forum.rclone.org/t/random-files-fail-to-copy-from-jottacloud/21448)
And one negative:
A month after I initially reported the issue I finally received a conclusion from support. Jottacloud had found 50 files that were corrupted and permanently lost.
(https://mpolden.no/blog/posts/2021-06-09-jottacloud.html)
In my case I have tried deleting some of the files that failed and re-uploading them, and that fixed it. I have left the rest of the failing files as is for now, because I'm curious to see if this is something temporary that Jottacloud will fix on their part..