Just to update this, in case anyone stumbles across the thread:
I have resolved the problem by exchanging all affected Gledopto GL-C-008 Controllers. The ones which constantly crashed all had firmware version 3.0.0.
Manufacturer / importing company told me, the ZLL (Zigbee Light Link) Controller should never have programmed with a firmware version 3.0.0 - latest supported version was 2.0.3 (at the time of writing).
Controllers running this version don’t crash at my place. I can confirm that also 2.0.1 and 2.0.2 are working as intended.