Cannot drop database because it is currently in use

Dear colleagues,

I use SQL Server 2012. The exact version info is:

Microsoft SQL Server 2012 - 11.0.2100.60 (X64)   Feb 10 2012 19:39:15   Copyright (c) Microsoft Corporation  Standard Edition (64-bit) on Windows NT 6.2 <X64> (Build 9200: ) (Hypervisor) 

I am flabbergasted. I try to drop a database, but that won't work because it says it is in use. But it isn't. I think I should no what to do, but actually I don't. Below, the SQL I use. Per statement, in the comment below, you can see the result. I cancelled the 'SET SINGLE_USER WITH ROLLBACK IMMEDIATE' after 4 minutes because nothing happened. This probably points to the real problem, but I cannot think what it is.

Kind Regards,

Chris Sijtsma

USE [master]
GO

EXEC msdb.dbo.sp_delete_database_backuphistory @database_name = N'Start10_64892_DB';
-- Commands completed successfully.
-- Completion time: 2020-04-15T07:09:32.7893007+02:00
GO

DROP DATABASE [Start10_64892_DB];
-- Msg 3702, Level 16, State 4, Line 9
-- Cannot drop database "Start10_64892_DB" because it is currently in use.
-- Completion time: 2020-04-15T07:10:10.1496410+02:00
GO

SELECT COUNT(1) FROM master.sys.sysprocesses p WHERE p.dbid = DB_ID(N'Start10_64892_DB');
-- Returns the value 0
GO

ALTER DATABASE [Start10_64892_DB] SET SINGLE_USER WITH ROLLBACK IMMEDIATE;
--      :
-- Nonqualified transactions are being rolled back. Estimated rollback completion: 0%.
-- Nonqualified transactions are being rolled back. Estimated rollback completion: 0%.
-- Msg 5069, Level 16, State 1, Line 10
-- ALTER DATABASE statement failed.
-- Query was canceled by user.
GO