SQL server DAC connection to mssqlsystemresource database
I just managed to connect to my SQL Server using DAC, where the SQL Server was started in single user mode. I verified single user mode by looking in the errorlog file.

Using SSMS is not a reliable way to use the DAC or single user mode. SSMS has a tendency to open several connections, and you never know which one comes first - even for a single query window. I used SQLCMD. I managed to connect using SSMS single query window as well, but again, sometimes it works sometimes not depending on the orders for the (background) connections you happen to get.

Use SQLCMD.EXE instead of SSMS.