I recently started having this issue as well. Can't seem to find a solution anywhere. I've done everything everyone else has done here to try to fix the problem, but no luck. It started happening shortly after I had initially texted someone and it DID fail to send the message, and I discarded the message because it wasn't important. Now every time I open an existing thread or try to start a new one, the "Message not sent" notification pops up. If I "Select it to review it and try again" and I'm in an existing thread, it will change the 'To' field in that thread to be empty, though all messages sent and received in the thread will still exist (this does not delete or change the existing thread in any way, it just clears out who messages are being sent to in the thread). If I "Select it to review it and try again" and I'm not in any thread, it will bring up a blank thread as if you'd selected to compose a new message. In both occurrences, it will give the "Undelivered message. Unable to send this message." warning, and then put the "Message not sent" notification back up in the Notifications panel. If you keep selecting it, it will keep putting the notification back in the menu. If you clear all notifications, it will reappear anytime a thread is opened (really annoying when you are just receiving and checking a text). All texting operations (sending and receiving) seem to be working just fine. You CAN prevent it by going into your message settings and unchecking the “Display message notifications in status bar,” but then you also don’t get the useful notifications when you receive new texts…
Any help on a solution would be greatly appreciated. I’d rather not have to completely reboot my phone. Thanks.