Fatal Error 2.0.2.29 "Object must be a root directory ("C:\") or a drive letter ("C")"

This error has been mentioned in at least two other topics so you’re not alone.

Would you be able to Export your backup job “As commandline…” and paste it here (with appropriate personal stuff like username, password, etc. removed)?

I’m wondering if your source folder parameter has a single slash after “C:” as I saw from another user.