"TASK MANAGER" process is taking over a single-user mode database. What is it?
Had the same issue today, If you have not put your AUTO_UPDATE_STATISTICS ASYNC OFF, you will not be able to enter your database, you can solve this by putting your database offline. Important to know is that you need to set your deadlock priority to high, otherwise you will be deadlocked out of the command. Use following commands to get out of SINGLE user mode

SET DEADLOCK_PRIORITY HIGH

ALTER DATABASE [YourDBName] SET OFFLINE WITH ROLLBACK IMMEDIATE


Followed by

SET DEADLOCK_PRIORITY HIGH

ALTER DATABASE [YourDBName] SET ONLINE WITH ROLLBACK IMMEDIATE


Followed by

SET DEADLOCK_PRIORITY HIGH

ALTER DATABASE [YourDBName] SET MULTI_USER WITH ROLLBACK IMMEDIATE