As soon as you add Script Editor Web Part to a page in SharePoint 2013 and publish the page, RSS Viewer Web Part will get stuck on the loading animation. If you remove the Script Editor Web Part, RSS Viewer Web Part start to work immediately.
NOTE! When in Edit mode, and even after checking-in the page, RSS Viewer works nicely, this only occurs after you publish the page.
None, will add it here as soon as I figure out a workaround. Occurs at least on March 2013 update, unsure if it occurs on RTM bits.