Error: 18456, Severity: 14, State: 46
Reason: Failed to open the explicitly specified database.
...
The Database which the login is trying to access is a temporary Database
and is in Single user mode for last 15 days failing with above error.

The error clearly says that the database specified in the connection string cannot be accessed.
So you have these possibilities:

change the connection string with master database
make your database accessible, put it in multi_user