How is this a UBB5 issue? You're using UBB6! <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" /> Movint to UBB6 PIT....
If ubb6_test.cgi says that the file is not there, then it is not there. Ensure that the directories exist and are properly cased, i.e. Net/SMTP, not net/SMTP or NET/smtp, etc.