We are currently running SharePoint 2010 Server SP1 + June CU. Our site collections were migrated using database attach method from SP2007. When we configure 2 or more XsltListViewWebPart that point to any list or library and they use collapsed grouping in views the hanging with loading... message when you click the expand icon. If we one have one XsltListViewWebPart and a group by set to collapse it works fine. Our farm consists of 3 WFE load balanced behind a Cisco ACE. Has anyone else ever run into this problem before?
Modify this web part -> Locate the "Enable Asynchronous Update" checkbox under Category "AJAX Options" -> Check it -> Verify