This error is due to an incomplete configuration of the SourceForge server. To correct this, add the following to /usr/local/sourceforge/sourceforge_home/etc/sourceforge_configuration.properties:
sf.requireAssociationOnDocumentCreate=true sf.allowedAssociationTypeOnDocumentCreate=Any sf.requireArtifactToBeOpenOnDocumentAssociation=false
After restarting the application you should be able to connect using SourceForge Explorer.