Didn't work for me.

I changed the settings discussed above for default database and log file locations to the actual location (on the D: drive) and rebooted the server, got the...

I guess I was not explicit. The Backup Exec backup job was NOT called by the Maintenance Plan, BE runs on a different server, and does a SQL backup completely...

Well, it's the native sql backup, called by a Maintenance Plan.

FWIW, the Backup Exec SQL backup ran just fine. (Yes, I'm a belt-and-suspenders kind of guy.)

No, I haven't tried running the command in t-sql.

In fact, looking at the error message in the history says 'cannot perform a differential backup... because a current database backup...

This is EXACTLY the sort of useless, deliberately obscure, error message that gives Microsoft such a bad name.

I set up a Maintenance plan to do an incremental backup of a...

An interesting article, but for the prejudice against name-brand servers.

While I appreciate that technical experience can make a 'white-box' solution more palatable, you run a huge risk of discovering that...