Dear Kaan,
http://help.sap.com/saphelp_nw70ehp2/helpdata/en/d6/63f6413a15e23ee10000000a155106/content.htm
You can do the following:
Try clearing the java webstart cache,this u can do by opening the JavaWebstart, go to File->Preferences->Advanced and click on Clear Folder.
and the try to open the repository again.
Also make the proxy settings to NONE in the JavaWebstart( File->Preference->General).