Hi! I had similar problem and it seems I've managed to solve it at last!
Solution was unexpected and not obvious, but simple:
1) In iMessage options change in "Send from" field your phone number to e-mail.
2) Try to send message - in my case it started online checking if it's possible to send iMessage
3) Change "Send from" back to phone number.
Please, share result - just interested if it's accidental or really works