Short update:
The “devil” is the openhab android app. See this topic: Slider items with new Android App 2.8.0 not working anymore
I just still didn’t get why tahoma kicks the bridge and the things offline when getting a float number instead of integer. But for now I can have a calm sleep because my rules with groups are good as they have been at the beginning.
Sorry for bothering you with a pseudo-problem. 
But at least I’ve learned more 
about iterating through groups and DP.