Since not every system is working as the other, I had to optimize my MyCPU build. At first I had issues with the register monitor causing frequent crashes of the MyCPU. Dennis thought about it and said that maybe the adress lines were too long causing reflections at the ends and the CPU adressing the wrong registers. I did some measurements and long and behold, the adress lines were the issue. The reflections sometimes peaked at about 12V which caused some buffer IC to exhale their magic smoke.
I managed to shorten the backplane bus to about half the length by connecting the modules to both sides of the cable. After that everything worked fine with fewer crashes.
After that I wanted to turn up the clock speed as fast as possible without the system crashing. The flow-chart Dennis provided for that was really helpful. I could tweak my system to about 7.2MHz without causing instabilities.
From time to time I catch myself powering it up and just play around in BASIC. It really feels nostalgic and I love it.
Here are some pictures of the assembled build and the register monitor.