I am not able to reproduce the problem.
I created 25 divs, each one I was able to rename.
My steps where to select the contentWrapper div in the tree, then click the plus button and select div. In the Name box I would change the name of the div, for testing i was using div<n> where n is a number. Then select the div above it in the tree (for example the content div) to see the name change take effect in the tree structure.
How many divs are you adding?
What are the steps you are taking?