Currently it's not clear what causes the issue, so I already requested the logs via email in support ticket. We'll continue communication there.

Hi Matt,

Thank you. I have sent the logs directly from CB app.

Please let me know if you see them.

Regards,
Bartosz

Bartosz, did you ever get this solved? I'm getting this error on server image backups and am having trouble nailing down what's causing it.

It's best to open a support case with us by sending us the logs via the Tools | Diagnostic toolbar option. I prefer to avoid sending instructions on a fix for another customer that may not address your particular issue.

Seems to be an issue on OS side: https://social.technet.microsoft.com/Forums/en-US/5b3886fc-77b0-4fbf-a1aa-083346e731db/error-0x80090006-invalid-signature-while-copying-files-from-windows-server-2012-to-windows-7?forum=winserver8gen

In one of the tickets the issue has been resolved without any input from our side and we didn't get any feedback in the other ticket. Still, I would suggest to synchronize the repository via tools > options > repository menu just in case.