When the rendering cache is turned on the admin pages do not render correctly. The main Xwiki admin page looses all its formatting. The Group and User admin pages keep trying to load without every actually loading.
The 'not so clean' workaround to that problem right now is to add refresh=1 query string at the end of every URL. I believe the cleaner approach would be to add 'admin' as an action to force refresh of the rendered content cache in DefaultXWikiRenderingEngine. Better still, the list of actions to force refresh (inline, admin, etc.) could be parameterized.
If I get some direction I would be willing to submit a patch for this fix.