Login failed for user '(null)'. Reason: Not associated with a trusted SQL Server connection.
It appeared nearly 20 times, then about 30 minutes after the first occurence, I noticed in my Event Viewer this error:
The registration of DNS name XXX.XXX.XXX.XX.in-addr.arpa. for network name resource 'SQL Network Name(SQLServer)' failed for the following reason:
DNS operation refused.
After that I began recieving hundreds of errors like this one:
Login failed for user 'SQL user'. Only administrators may connect at this time.
I was unable to connect to the server to see what was going on so I had to failover the server. That's why my troubleshooting is limited to error logs. After the failover, everything came up fine with no more errors. Has anyone ever encountered this before?