Not broken for me in .51
If you changed your web server path after you installed the update, then BI probably installed UI3 to your alternate location.
Just about to re-run the .51 update.exe and right before I pointed the www back to its default folder, and restarted the service just in case. Guess what, UI3 works fine now, I renamed the UI3 github file folder to make sure. It still works.