Both mount command and systemd file look OK.
It looks like some kind of transient error or rate limiting performed by pCloud.
Perhaps you can find some useful tips in this somewhat similar thread:
https://forum.rclone.org/t/how-to-handle-pcloud-rate-limit/31279
My best advice is to test using a very simple copy command like described here:
https://forum.rclone.org/t/how-to-handle-pcloud-rate-limit/31279/5
You can add flags:
--tpslimit 10 --tpslimit-burst 0
to your mount command. They control speed rclone "talks" to pcloud. And keep decreasing tpslimit until errors are gone.