Someone wrote this and it seemed to work...
If you go to stock messages app and turn off advanced messaging in the app's settings. Restart you device.
My phone just started this we have tried everything even factory reset. Any luck
Have you reached out to your carrier regarding this yet?
It's definitely the AT&T cloud sync for the messages. I turned the sync off and the couldn't connect to server went away. I have had it since the last update and after that it was gone