The solution is simple. Check if you modified the IgnoreUrlPrefixes setting according to the SDN5 instructions:

*Sitecore 5.1 *instructions:

|/sitecore modules/forum/web/|/users/avatar.aspx|/rss.aspx

*Sitecore 5.2 *instructions:

|/sitecore modules/forum/web/|/users/avatar.aspx|/rss.aspx|/WebResource.axd

The thing is that the *GroupRepeater *control relies on the ItemCommand event which is not simply fired unless you add your page to the IgnoreUrlPrefixes list. This is applied to 5.1.1.x. Not sure that same problems exist in 5.3 though.