Unusual SqlException when starting a connection to SQL Server

I have several clients who can connect to our database without any problem. One of our clients had to update our program after he received a new computer and was unable to connect successfully.

The connection process includes a stored procedure that validates the username, password, and some other data and returns true if this user is allowed to use the database. Stored procedure call is an error.

System.Data.SqlClient.SqlException (0x80131904): The connection was successfully created with the server, but then an error occurred during the login process. (provider: SSL provider, error: 0 - Das format der empfangenen Nachricht war unerwartet oder fehlerhaft.)

This last bit is in German - it translates into

The format of the message received was unexpected or incorrect.

I've been tearing off the others for a long time. Here, the rest is like an image. Sorry - I don't have it as text.

I've already tried

• with user reboot.
• restarting the SQL server
• checking that TCP / IP, named pipes and shared memory are enabled on the SQL server

SQL Server Version Information:

Microsoft SQL Server 2008 R2 (SP2) - 10.50.4000.0 (X64) Jun 28, 2012 08:36:30 Copyright (c) Microsoft Corporation Express Edition with Additional Services (64-bit) on Windows NT 6.2 (Build 9200: )

Everyone else seems to have no connection issues as usual, but this is only one client.

Now, one thing that is odd is that the ISP is the "SSL Provider". As far as I know, I am not using SSL. Can the client read it as SSL when it is not and what is causing the unexpected data?

I'm running out of things to try - any ideas on how should I proceed to troubleshoot?