Some feedback on my issue…
I saw that the Zigbee channel was 15, which was the same band as my WiFi (created by my mesh accespoints (Dece M5)). I changed the Zigbee channel to 25, reconnected some devices, and now all my issues have moved to the past. No dropped Zigbee devices anymore! Thanx for the help.
But… I have several Ikea Styrbar Remote Controls. With the Hook collection blue-print those functioned like a charm. But now I cannot get them back into action. Anyone an idea what is going on? I do see them in my Zigbee network, but they don’t control anything.