Thank you for the replies. Very helpful: Since it worked for -well- at least some of you, I reasoned the issue couldn’t be a generic bug, but had to reside in my setup.
For those who find this topic while searching a solution for the same issue:
After some useless tinkering, I stumbled upon the nature of the problem (Don’t overestimate my debugging capabilities: Pure luck):
I just added all related network devices (Homey, MiLight bridge and for good measure all other domotica) to the DHCP reserved IP list (IP address x should always be assigned to Mac address y") in my router’s settings.
Bit crude but effective: The MiLight app/related devices started working, but were a bit wobbly initially. After a few restarts of the app everything is completely stable & working fine.
I still don’t have a clue what the root cause was, but this seems to fix the problem, so worth trying.