Without seeing your .cfg file it's difficult to guess. Unless you have moved (in your .cfg file)
$db_debug = 1;
to up above the section
# File and URL's
it will not always display errors within that file itself. Try moving that line up and see if you then get an error message.
Have you made any changes in your .cfg file? You might also want to double check and be sure your /auth directory has the permissions set at 777.
Hope this helps
Unoffical DBMan FAQ
http://creativecomputingweb.com/dbman/index.shtml/