I am on a tablet at the moment so I logged into the router to try the update to give you a screen shot of the error and it updated I am totally amazed, so it may depend on different operating systems or browsers, but you have hit on another good point because it uses an old version of LuCI, I know a few other people have had this issue so I would recommend them to try a different browser once you have check you have available space left.
Thanks for your help