For some unknown reason, an MSSQL server will not give you a useful error message when you try to access it without the right permissions.
When you're user user is not allowed to access ressources on the machine on which the MSSQL server is running on, you won't be able to connect to the database as well.
Now here's a quick and dirty workaround for this situation:
---- variant 1 (for click-lovers)
1) open windows files explorer
2) select the MSSQL server machine in the network neighbourhood
3) connect to any ressource and logon using credentials with sufficient rights
4) try to connect to the database again. now it should work.
---- variant 2 (for shell lovers)
1) open a dos-shell
2) type: "net use X: \\mssql-server\c$ /user:db-user"
where "db-user" is the user with sufficient rights