If there's multiple addons in the lower right that you can't use the mouse in that same region, then it's likely a transparent mouse active frame that's overlaying it.
You can try the following script to see if it will identify the frame by placing your mouse over the area that doesn't work, then typing in the following command...
/script print( GetMouseFocus():GetName() or "unnamed frame" );
If it prints a name you recognize, you can do something about it. If it prints a name you don't recognize, post here and we can try and sort out what it is.