Cannot Replace World

I can replace the world with a blank uploaded world, but if I use even the simpliest addon, it fails.  If I try to add any addons, it either says nothing and goes back to the same page or it tells me some items couldn't be applied, which means nothing was applied.  HELP!!Unable to render embedded object: File (  The local world is a little less than 40 MB if that makes a difference, and what's funny, is the addons that fail to add are the same addons that are already on the current realm...though, I also cannot view the addons on the realm for some reason either.  HELP) not found.!!