Hello Alex,
Thanks for the quick response 
I did these tests with two Masters without any more icinga instances as well and got the same results.
When one icinga instance goes offline (gets stopped) everything is handled fine and the remaining instance takes over all the checks.
My problem is when the instance is running, but isn’t able to reach the client due to network issues between them (icinga and client) and another instance in the same zone (doesn’t matter if master or satelite/agent) is able to reach the client because it is able to use a different path in my network.
Regards
Jo-Jo