What do you mean, you can't edit the bottom right corner? What kind of editing are you attempting, and what is happening when you attempt it? Please describe the steps clearly.
If you mean the brush cursor changes to an arrow cursor in part of the photo than this is a bug/glich that has been in Lightroom for several versions. It has happened to me once or twice and a restart of lightroom solved the problem. It seems like there is some kind of hidden dialog box behind lightroom or something.