For example, "No access to 1 files / folders (code: 1601)".

https://kb.msp360.com/standalone-backup/general/access-denied

Please review the KB article.

Error 1601 and 1610 are the same?

It's an access denied. The service account backing up cannot access a file. You can check the file from backup history and change its ownership of needed. But in general if the service account doesn't have access to a file owned by a different user account, it can't be backed up. Using the backup operator account can help prevent these issues. Or just check the file to see what's going on.

Actually, the issue is that the client renamed a folder that I had specifically included in the Backup Source list.

I see. So a specific folder was selected in the plan, and that folder was renamed, and therefore you got an error. Can I assume that you were able to figure this out from the history of the backup plan? If so, just let me know. Thanks.

Yes, the specific folder was mentioned in the detailed report.
I neglected to check the contents before posting in this forum.
My apologies.

No need to apologize. Thanks for the update and glad your issue is addressed.