Thanks for the answer Jon.
I also got some answer form mogodb community and it solved my problem.
Apparently the mongodb_uri that I was using for years is wrong and it should not work 
. Now I tried on non working node with new mongodb_uri and it connected right after restat to the cluster, all seem s to be working…
I would have to check and test with some older version, but it could be that a bit older veriosns of graylog somehowe knew how to correctly read previous “not correct” mongodb_uri.
The correct mogodb:uri should be:
mongodb_uri = mongodb://192.158.20.100:27017,192.158.20.101:27017,192.158.20.102:27017/graylog?replicaSet=reproduk&retryWrites=true&w=majority