Thanks for quick responses. This happens on startup, and again on next startup if I delete them. I also get the question if I want to import from my previously installed version (11.2), which it finds from ~/.netbeans/11.2/. I see that folders ~/.netbeans/12.0 and ~/.cache/netbeans/12.0 have been created, which sounds like plausible destination for such config files. Where would this XDG folder/path be?
$ env | grep XDG
I can also add that at first it did not want to start, but after I moved /usr/etc/netbeans.clusters to /etc/netbeans.clusters it worked. Not sure I understand fully what impact this has yet, perhaps related.
Edit: The reason I copied this file over was because I got the following error when trying to start the application initially:
Cannot read cluster file: //etc/netbeans.clusters