Hello eitan,
are you sure, that no other users or applications use the server while using DataXport?
If the Excel file was created by DataXport, I think the problem is caused by the log or the .ulp file.
Another idea: Reinstall DataXport. Before installing DataXport again make sure, that the server is "DataXport clean".
This means:
1. all DataXport-Folders should be deleted. --> save your files before deleting!
2. the registry is free of entries of DataXport --> check this manually or with tools, e.g. "ccleaner"
After reinstalling, create a new .ulp file. Do not use an existing .ulp file for your test.