Dear Martin,
I find out the reason. Someone told /write me here, that I must install the 64bit for RASPBERRY PI4 - Plattform to using the new 1.6.x - Version from CheckMK. But this information is /was wrong.
On the github - website is ONLY the CheckMK - Version for armhf & this is a 32bit - Version 
Now, after all tests, etc. I install now DEBIAN BUSTER for armhf, install the newest Version from the github & it´s work
Thank you very much and best regards
Zhéxué M.