This has been happening to me too, changed the graphics card to see if that was causing it but it still happens.
Likely the same cause so I'll be keeping an eye on this thread.

Did you reply to "All"?

How did you manage that?

Took out Ken's name and typed All.

Meant to be to everyone but didn't realise All was a user.

(Opera mobile on my Android if it's not meant to be possible and needs fixing.)

You don't need to type anyone's username if it's to all, just leave it blank.

There should also be a "Reply to All" link at the bottom of the messages page, which will do it for you.