When a user can't access the ClassLive tool and gets the error 

"Error

An error occured while processing the request.

Your invitation to attend this session has been revoked."

you can try two simple tricks:

1) Reset the ClassLive tool - obs: Please only do this when there are no recordings. If there are recordings and you reset the tool, you will loose all your recordings permanently.

or

2) Remove the participant that has problems from the members list and add it back.

In case none of the two tricks helped, please create a ticket and we will have a look at it.