System.Data.SqlClient.SqlException: \'A connection was successfully established with the server, but then an error occurred during the login process. (provider: TCP Provider
I tested the connection between UWP app and SQL server 2008 R2, and can reproduce your issue. I also tested the connection between UWP app and SQL server 2017, with the same code snippet, same configuration for SQL server, it worked very well. So that as a workaround I strongly recommend you to upgrade the SQL server to 2017 since SQL server 2008 R2 is already out of date that not supported.
Additionally, don't forgget to Enable TCP/IP Network Protocol for SQL Server which you didn't mention it in above description.