Could you check (with Firebug for example) which request is failing and what the response of the failing request contains.
What do you have in episerver.packaging section in web.config?
Could you try to check the POST request and URL when add-on system tries to read the list of installed add-ons? The URL should look like http://siteurl/pathtoUI/AddOns/AddOns/GetInstalledModules?moduleArea=EPiServer.Packaging.UI
Fiddler, Firebug or Web INspector should be good tools for that.
I've just installed a CMS7 on my Win2008R2 envirionment, and the add on store doesn't work. When accessing it through the site, it just keeps "flickering" and reloading the page with the error message "Error reading add-ons list Syntax Error: syntax error, unexpected token <". The page is reloading with a 1 second interval, so I need to go to another URL.
Is this something anyone have experienced ?