Unable to submit Pending Files (no error message)

About a week ago the file-submission facility stopped working.

I am able to Lookup pending files in the normal way, and receive a response; the Purge facility is also working properly.

However, when I click ‘Submit’, the message-window that appears informs me that the files are being prepared and then submitted, but then disappears without showing me any progress-indicator. And I do not receive any reponse from Comodo.

Comodo Firewall Pro version
Windows XP Home Edition SP3 (32-bit)
Other installed security software: Avira AntiVir PE Premium; PC Tools Spyware Doctor; Blacklight Rootkit Eliminator; NoFlash; NoScript; EasyCleaner; RegSupreme; MaxRegistryCleaner; CCleaner