I just solved it...
I can slap myself for not trying this/thinking about it:
The monitor was connected via DVI plug (a VGA to DVI connector, plugged into my DVI port on Geforce card). This has always worked, and, as far as I know, I also got it to work on the LXDE full version. I certainly did NOT use the VGA connector into the VGA port directly (as is necessary now).
My guess is that now any distro will just work fine with my monitor.
Thanks everybody who tried to help. But this does raise the question whether DVI monitors are OK for Linux if they have no VGA option?