“A connection could not be established to . Reason: Login failed for user <server\login_name>. Reason: Server is in single user mode. Only one administrator can connect at a time.”</server\login_name>
This problem may occur when you try to start the SQL Server service by putting “-m” as the startup parameter, and then restarting the SQL Server service.
The “-m” parameter starts the SQL Server service in single user mode. Therefore, only one admin can be connected at a time.
Because SQL Server starts in single user mode, if your SQL Server Agent or another service (which usually becomes SQL Server Agent) connects to SQL Server before you, you will not be able to connect to your SQL Server Instance and encounter this error.
Additionally, in SQL Server 2005 and later, SQL Server Startup parameters can be changed with SQL Server Configuration Manager. So to change this parameter, you don’t have to connect to SQL Server Management Studio.
If this problem occurred in a Cluster environment, you must clear this parameter from all nodes.
You can find detailed information in the article “SQL Server Startup Parameters“.