My setup: A new (2019) MacBook Pro 15' running Mac OS Mohave (10.14.5) with a new 4TB WD My Passport. I had my first complete backup yesterday, and about 20 successful hourly backups since then. It took 3 calls to Apple Support, but they have solved it for me. I had exactly the same problem - no matter how many times a I rebooted my Mac or reformatted my WD drive, the first Time Machine backup would succeed and any subsequent one would fail with “Error occurred while creating the backup folder”.
