Issue – Login failed for user ‘xxx’. Reason: Server is in single user mode. Only one administrator can connect at this time.

Cause – Ensure SQL Server is not running in Single user mode by checking start-up parameter.

if you see -m; it mean your SQL is running with single user mode.

-m : Starts an instance of SQL Server in single-user mode. When you start an instance of SQL Server in single-user mode, only a single user can connect, and the CHECKPOINT process is not started. CHECKPOINT guarantees that completed transactions are regularly written from the disk cache to the database device. (Typically, this option is used if you experience problems with system databases that should be repaired.) Enables the sp_configure allow updates option. By default, allow updates is disabled. Starting SQL Server in single-user mode enables any member of the computer’s local Administrators group to connect to the instance of SQL Server as a member of the sysadmin fixed server role.

Resolution – In order to resolve this issue; you need to remove -m from Stat-up parameter and restart the SQL Server.