[aiharkness]

It's been a while for me and I think I have forgotten something. Make sure your calendar (CICAL) and messaging (CMIME) in Default Services in Advanced System Settings are both set to an email address and the same address for both. If that is not the way it is set now, that very well may be the problem.

The part I'm not sure about is why it works sometimes. If you have more than one email address set in BIS, see if the invites that work are all sent to one address and the one's that didn't work are sent to another. If all invites were sent to the same address then we need to look deeper.