I was asked to check a clients 2008 r2 SQL Server recently because there were high numbers of the above error in the SQL Server errorlog.
I initially suspected a broken Database Mirrored setup but on checking the server, I could see no Database mirroring, no Availability Groups, no Log shipping setup. Checking the Server Objects > Endpoints in SSMS, I found a ‘stray’ endpoint, which looked suspicious.
I scripted this out and could see this was listening on port 5022, which is a port typically used by AlwaysOn Availability Groups replicataion.
As AOAG was not configured for this server, I was able to simply drop the Endpoint.