I think I tried this, but it is necessary to open the board and/or module to measure voltage etc. pp.

It is a common approach to look at the end of the current path and measure the voltage. Then you go backwards and check were the voltage drop occurs.

I'm afraid it is not only to to put a PCB with voltage regulation into the München board. There must be some problem with wiring inside the board. Otherwise the MM IV modules should work with 5,5 - 6 V at the power connector...