I tried to compare a file, with several changes, across patchsets 10 and 11. That didn't work.
See the link. I get essentially a blank page, the menus indicate that Gerrit thinks I may be in a search result page or something--definitely not a diff.
hashar suggested that I should uncheck the syntax colouring option, but that changed it into a server error. Very annoying.
Dunno what's happening here, but clearly it's not good.