Error 108?
Assuming it's intermittent:
Someone else has it open exclusively.
Citrix\RDP scenario - are there 'ghost' instances of your EXE still running on the server from remote user sessions that didn't terminate properly, that might have the file open ?
On-access antivirus scan is having a look at the DBF.
Windows Indexing service on some versions of Windows Server is having a look at the DBF.
Continuous online backup software like Mozypro is having a look at the DBF.
When it happens I would tend to hop onto the server and have a look at what sessions have what files open.