Weird flapping behaviour

CMK version: 2.1.0p4

Today I encountered that weird behavior: There are about 15 Hosts that are for real down, but 15 more are constantly flapping (the state/amount of flapping hosts changes every time I refresh the page). I had the same problem yesterday (spike in the graph) which I fixed by restarting the page. Today this did not work. All the hosts that are flapping, I can ping normally through my PC and through the server checkmk is running on with very low latency.

The cmc.log just says:
[fetcher pool] [service "****;Check_MK"] [helper 2992993] aborting running check
I guess because it’s considered down?

Checkmk fetcher usage: 50%
Checkmk helper usage : 17%
Checkmk checker usage: 22%
Fetcher helper usage: 33%

Does someone know how I can troubleshoot this further?

Maybe the restart did work. At least it settled down for now and everything is back to normal.