Messages app freezing after installing Sonoma

I have a 2019 Intel iMac. I just installed Sonoma this morning. Almost everything seems to work ok (except things are quite slow at first, I hope that they'll get better). However my Messages app, which was working just fine before the upgrade, is now frozen and unresponsive. I've left it sitting there for 1/2 hour, but still nothing. I've tried force quitting and restarting, with the same result. I even rebooted (which is a bit painful, because it takes almost 1/2 hour until everything is running ok) with the same outcome. Are there any suggestions? Since Messages is included as part of the OSX I don't think that I can remove the app and reinstall. Can I?