This is the actually the same as my thread at http://forums.arcgis.com/threads/101596-Microsoft-JScript-runtime-error-xhrFailed-when-switch-V2.5-to-V3.5. But this one has a better title to describe the problem. When I switch JS V form 2.x to 3.x (e.g. 3.80), I got Error 502 - Bad Gateway at forums.arcgis.com//ajax.php?do=quickedit&p=361501. How can this be solved? Thanks.