The database backup in WFBS 9.0 with SP1 and SP2 installed failed.
The problem occurs because the OfcDBBackup.exe was not replaced to SP1 version during upgrade. The OfcDBBackup.exe is missing from SP1 and SP2 small packages. The issue will not happen if you are using SP1 full package.
To solve the issue, you need to replace the OfcDBBackup.exe file with the OfcDBBackup.exe SP1 version, which is Build 19.0.2261.
File path: PCCSRV\Web\Service\
Note: Stop Master Service before replacing the file.
File to replace: OfcDBBackup.exe
New version: 19.0.2261