Backing up to and restoring from NAS: Failed to process file

I’m always getting three errors when backing up (and now also when I tried to restore it, unsuccessfully, and then when trying to verify the files):

2021-08-17 17:39:54 +02 - [Error-Duplicati.Library.Main.Operation.TestHandler-RemoteFileProcessingFailed]: Failed to process file

I went to the detailed log in the about section and got this:

Aug 17, 2021 5:39 PM: Operation Get with file attempt 1 of 5 failed with message: Invalid handle to path "/mnt/mount/NAS/EXTERNAL/BACKUP/xxx/"

However, that file actually exists under that path, it’s 50M big.
It tries to get that file but aborts after 5 attempts.

Also, upon every backup / restore / verify attempt, it gives me three new file names… not always the same.

What can I do?
Repair didn’t help of course…

EDIT: On another machine, file access and restore works. In the meantime, I also found out that AES Crypt for manual decryption does not work on the original machine. So I suspect it must have something to do with how the connection to the NAS is established…