A lot more UI changes are going on right now on Test, than just the Merchant's Window.
I would say that 99% of us cannot load ANY Custom UI's since a patch on Thursday night. I know that I have tried all of the UI's that I had stored, and I cannot even load any UI's that have been updated/posted here in the last day or so.
Other than the merchant window, the only change I've noticed is a new window for the address book.
This has far-reaching consequences though. The new window needs to be mentioned in SIDL. The email window needs to be modified to make use of the address book. The options window has new address-book-related options. The selector window seems to be getting an address-book button. And then Animations needs to include the new graphics for the selector button.
So... many UIs will need significant updates soon.