I'm also in the dashboard preview. I've had a similar issue since early November, but I don't know if it's exactly the same. It comes and goes.
- I can send messages to other people, (party and game invites, for instance) and they get them, but trying to respond to them just times out.
- If other people send messages to me, I can't receive them. If I log out and log back into Xbox Live from the Xbox menu, then I'll get a flood of messages all at once.
If I manage to get one of these messages, and the lobby or party is still open, I can join the lobby/party. Usually when I've identified that this is the problem, I'll ask my friends to start a lobby and send me a combo party/game invite, and then I'll quickly log out and back in, and accept the invite.
Sometimes this happens in reverse (ie, I'm the one who can get messages that I can't respond to), and the workaround is reversed. It requires a decent amount of patience when this happens in order to get to play with my friends.
If this isn't your issue, or it doesn't solve it, you'll want to talk to Microsoft via the Connect Dashboard. You should have access to this page if you're in the dashboard preview. From previous experience, I don't think they're able to roll back your dashboard revision, but you might find a solution or workaround, and at the very least, we're helping everyone else avoid this issue when the update goes live.