I checked for network issues, I can normally ping and access filer.
In snapdrive I can see all drives, snapshot etc. everything looks normal
In our network we don't use DNS to communicate with filer, we use RPC (In snapdrive in "transport protocol settings" I have RPC and Filer IP)
I can create snapshots, view them and delete. So everything looks normal.
This warning shows only from time to time and I do snapshots every hour.