@sitwon thanks for trying to help, but this is a different issue reported earlier today in #1641: There’s a file in the repo which does not have a valid name (the name is not a SHA-256 hash). Before, restic ignored these files, but since the last internal change this bug got introduced. It’ll be fixed with #1643.
If you like, you can have a look which files have an invalid name with the following command:
$ find /path/to/repo -type f | egrep -v '/[a-f0-9]{64}$'
The files can probably be removed, then the error will be gone.