Thank you for the quick reply!
That worked great! In case someone else comes along here, this is what we did in detail:
kopia snapshot verify --all-sources --verify-files-percent=100
gave us the following output:
Found 4 objects, verifying 1, completed 0 objects.
failed on root@osboxes:/home/osboxes/Desktop/source@2021-03-14 13:37:01 EDT/horoo.jpg: error reading object Z385d923526dea28c53a3a23ad9179eb1: unexpected content error: invalid checksum at pb914b2361932491cb17acf7f82d22050 offset 754 length 107799: decrypt: cipher: message authentication failed
kopia: error: encountered 1 errors, try --help
which revealed the content ID: Z385d923526dea28c53a3a23ad9179eb1. We then needed to remove the Z at the beginning and use it to remove the content object:
KOPIA_ADVANCED_COMMANDS=enabled kopia content rm 385d923526dea28c53a3a23ad9179eb1
After that, we created a new snapshot using the --force-hash=100 option:
kopia snapshot create /path/to/source/ --force-hash=100
This recreated the corrupted file and even the first backup could be restored again. 
