For Chromium: does it help to remove the .chromium in your home-dir? (attention: backup of your bookmarks, preferences, ....?)
It worked! I renamed
.config/chromium to
chromium_old, started Chromium and it opened up fine. So renamed
chromium_old back to
chromium again, restarted Chromium, and it's there with all my settings intact. I don't quite understand why, but it worked

Thanks again all!