Update error

Every time I try to update my version of Vassal I get the following error;

An error occurred while launching/running the application.

Title: VASSAL 2.6
Vendor: Rodney Kinney
Category: Download Error

Found unsigned entry in resource: (http://www.vassalengine.org/ws/VASSAL.jar, 2.6.1)

Any help would be greatly appreciated. Thanks, Derek.