Hotcopy fails with error "You cannot use file :\PX ..."

Hello together,

with the Backup using the HOTCOPY command I get the following error in the hotcopy output file::

Backup W1 5.0 SP1

Backup Tool for Microsoft Dynamics NAV version 5.0 SP1
Copyright (c) 1987-2008 Microsoft Corporation.

Backup started at 2010/02/5 02:00:01.

Free space on the destination volume H:\ is 193231540 KB.
Total size of database files is 50000000 KB.

List of database files to be backed up on volume H:\:

List of backup files for volume H:\:

The following message came from the server.

You cannot use the file :\PX because it is being used by another process.

Backup ended with an error at 2010/02/5 08:11:16.

Usually the backup starts at 2AM and takes around two hours. This one failed at 8:11AM with the error as shown above.

Any suggestions would be appreciated.