It is recommended that you upgrade the Domino server to a new version before the clients. This is because there may be features in the new client that are unsupported by the older Domino version. In your case I would suspect that that is the cause of your problem.
Setup a meeting from a v6x client and invite a v7 client user and then have that user accept the meeting. This is where i am getting and the error message i mentioned in the original post.