Trouble Shooting "Unable to complete login process"
Troubleshooting this wasn't the simplest of processes, as it turned out to be a third party application killing the user. However there were clear indications in the SQL log that there were problems.  

There were around 20 memory errors a day in the SQL Server log along the lines of "A significant part of sql server process memory has been paged out. This may result in a performance degradation. Duration: 7254 seconds. Working set (KB): 12756, committed (KB): 61520, memory utilization: 20%."

Using a product called ExtraHop (You can use your favorite network packet capturing tool) we were able to trace all the traffic going into and out of the server during the time frame of the 17052 error. This showed us that our monitoring software was connecting to the server and that a simple SELECT 1 was taking two seconds to complete.  A Change request was raised to increase the memory and then implemented and the errors went away.

I believe the monitoring software terminates its connection attempt to an SQL server; as the termination was coming from outside of SQL it would indeed be an unknown error.