Back up problem

I have a windows 2000 server running Exchange 2000 and I use Veritas Back up exec 8.5 with the Exchange lic added. I have been recieving a failed back up message, and in the Log I have these lines
The item \Program Files\Exchsrvr\MDBDATA\E00.log inuse – skipped.
The item \Program Files\Exchsrvr\MDBDATA\E00tmp.log in use – skipped.
The item \Program Files\Exchsrvr\MDBDATA\priv1.edb in use – skipped.
The item \Program Files\Exchsrvr\MDBDATA\priv1.stm in use – skipped.
The item \Program Files\Exchsrvr\MDBDATA\pub1.edb in use – skipped.
The item \Program Files\Exchsrvr\MDBDATA\pub1.stm in use – skipped.
The item \Program Files\Exchsrvr\MDBDATA\tmp.edb in use – skipped.

I have set the network access time to 10 pm (to log off everyone)and my back up runs at 11 pm.
What could be the issue?