Hi @wernerh, let me see if I can help out here.
Have you tried clearing the data for the messages app? This will not erase any previously received messages, but it will reset all settings for the app which might do the trick. To do this, find and tap Settings > Apps & notifications > App-info > Messages > Storage > Clear data > Tap "OK" > Restart the phone.
Another thing that you can try is to reboot your phone in safe mode to see if you are able to use the messages app there. All apps downloaded from Play Store is temporarily deactivated while in safe mode and by doing this, you can see if this behaviour is caused by a downloaded app. For more information on safe mode and how to use it, click here.
Try these measures and let me know how it goes!
