Additionally, I have a broken m-board (h-board shows up in different, even multiple slots). Is there something we can do about that?
There's nothing wrong with your M board. You're just using the wrong rPi image for your version of the M board.
I tried different rPis, different images, different m+h-boards, its definitifely the m-board. When I plug in one h-board (which works normal on other m-boards), this one board shows up in multiple, different slots at the same time in chainminer, with hashrates around 2 GH/s or less each and going down to 0 after some minutes.
At the moment I found one slot in which it runs more or less normal at 20 GH/s for about 15 minutes and then goes down to 0, so I made a cronjob to reboot the system every 15 minutes.
I had a h-board on which the regulator blew up while sitting on this m-board, since then its behaving strange, I suppose it was affected as well in some way.