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The documentation link you provided is very old, for SQL Developer 1.5.x. The problem is that the jtds-1.2.x.jar versions are for use with applications requiring Java versions prior to Java 7. SQL Developer 4.0.x supports Java 7 and Java 8, while 4.1.x requires Java 8. Please use jtds-1.3.1.jar instead.
The notification of which Java versions the various jTDS driver builds support is... jTDS - SQL Server and Sybase JDBC driver / News
In this forum, an earlier discussion touching on which versions of SQL Server we support, and which jTDS to use is... What versions of SQL Server are supported by SQL Developer 4?