I had to have same issue on my SQL Server 2016. When I check SQL Server ErrorLog. I saw that my service user cannot connect to one of the database.

Login failed for user ''. Reason: Failed to open the explicitly specified database 'TEST2'. [CLIENT: ]
Error: 18456, Severity: 14, State: 38.

When I check this database it's state was recovering. When I delete this database (It was not important for me), the problem has been fixed.

I had to have same issue on my SQL Server 2016. When I check SQL Server ErrorLog. I saw that my service user cannot connect to one of the database.

Login failed for user ''. Reason: Failed to open the explicitly specified database 'TEST2'. [CLIENT: <local machine>]  
Error: 18456, Severity: 14, State: 38.

When I check this database it's state was recovering. When I delete this database (It was not important for me), the problem has been fixed.