I've now started up JBoss AS again and trying to reproduce, but so far no "luck". If I change a file, then there's the message "1 files modified in module my_main_web" and nothing strange seems to happen.
For the moment, it seems that the sequence of steps of stopping JBoss AS, closing the projects and re-opening it, doing a clean and publish and then changing files while JBoss AS is not running, brought it back into a normal state.
One thing I forgot to mention is that the Server Log earlier also showed a main list with a heading called "Unknown Event Type", with only entries having nothing more than "OK" beneath it.