Hi,
Does your Application Builder work on the same computer as your Server? If not, you need to refer to the server name properly as well as make sure the network connection between those machines can be established.
If it's on the same machine, try deleting the web browser cache and try again with the new web browser session restarted. Make sure you can reach the service from Services Directory, make sure the security configuration of the map service is adequate. If no help, try restarting ArcGIS Server service from the Services in Windows and finally the machine itself.
Can you test using "http://localhost:6080/" in the Application Builder when browsing for services? Try using the machine name instead of the localhost.
Thank you Alexey for the very useful information. It works fine now.BestJamal
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Jamal Numan
Geomolg Geoportal for Spatial Information
Ramallah, West Bank, Palestine