Invalid Invitation Code

Hi,
Just a quick query if I may??
Occasionally we have people that try to log into our remote presentation and they receive a message saying "Invalid Invitation Code". It doesn't happen often and sometimes it is cured by getting them to try re-entering. I have just had an instance where that didn't work. The attendee wasn't using Avast which is sometimes an issue and was using firefox. Any ideas??
Thanks Heaps
Andrew Reekie