Plugin failed to connect to development mode server AT RETURN FROM PAYPAL PAYMENT

Hi, 

I have the problem in the object....

I create web application using Eclipse as Editor and GWT as Framework and I connected my app to PayPal SandBox. Now when I click "Buy" on my web page, it correctly opens PayPal web page to start purchase procedure, if in this page I change idea and I click "cancel and return to facilitator account' s test" , it returns to my web page BUT if I complete purchase procedure and at the end I click on "return to facilitator account's test" ,I have this message : 

"Plugin failed to connect to Development Mode server at 127.0.0.1:9997
Follow the troubleshooting instructions at http://code.google.com/p/google-web-toolkit/wiki/TroubleshootingOOPHM" 

code fragment : panel.add(new Hidden("cancel_return", "http://127.0.0.1:8888/App_negozio.html?gwt.codesvr=127.0.0.1:9997"));
panel.add(new Hidden("notify_url", "http://127.0.0.1:8888/App_negozio.html?gwt.codesvr=127.0.0.1:9997"));
panel.add(new Hidden("return", "http://127.0.0.1:8888/App_negozio.html?gwt.codesvr=127.0.0.1:9997")); 

can you help me to solve it? thanks =)

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