Well, I think you were Right with the increase of the resistance of the Heated Bed at higher Temp.
I connected the original Power Supply to the Mainboard and the HB Power Supply only to the HB. All Temps are now working:)
However, since this is an original upgrade, installed following the manual, no other upgrades installed, should'nt the delivered Power Supply not be strong enough?
221W max 24VDC 9,2A Meanwell
Mainboard 1.5.7
HB board v1.1