Can you post the connection string from your application? Is SQL Server on the same PC as your application?

>Thank you for explaining the default database, so it does not apply to my case. I have two users on this PC but almost always (99.99%) use one user. What I did, after I had the error this morning, is restarted my PC. The problem didn't go away. So I connected to the database using SSMS and had no problems connecting to the database. After that my application also had no problem connecting.
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