Historically modRana did not check that what id downloaded was actually a tile image. And some tile servers return a HTML error page when they can't return a tile (too low zoomlevel, too many tiles downloaded, generic tileserver error,...). So it could happen that modRana would download something that was not actually a tile image and then after coming to the area, it will try to show the image and fail - and because the "image" is already downloaded, modRana won't try to download it again. But quite some time ago I've added a check, that tests downloaded tiles and only saves them if they really are jpg/png/gif images (possible thanks to the magic numbers embedded in header of these image file formats). If you are using some old tiles, it could be there are still some non-image-tiles present - or I forgot to apply the check in one of the tile download methods.