warning: Cannot contact primary

Hi everyone, I have the error below on my mongos server and as I can see the balancing is not working anymore. This is an old version 2.6 installation with 20 Replica Sets, 3 config srvs, 3 mongoses. Any ideas what I should do to start the balancing again, please?

2020-12-12T01:27:36.463+0200 [conn547438] warning: Cannot contact primary for rs11/, to check shard version. The local replica set view and targeting may be stale.