Text message notifications stopped showing up two days ago which caused me quite some complications. With opening the app now, the messages showed as unread, but I didn't get notifications, so I missed them.
Last time I had notification issues it was caused by the MS exchange account which I was unfortunately forced to use again for a month or so now.
UPDATE: The problem is persistent, no text message notifications arrive, and there is no message sound either. Rebooting the phone later solved the problem, but I think this is a serious bug affecting basic phone features.