I discovered the hard way when I was trying to use Droid Explorer, that the SDK file structure has been rearranged.
In the case of Droid Explorer (a product that allows you to edit and build through access to the system files via a PC desktop), it was pointing to a file in SDK in the platform-tools folder, which has been moved into the build-tools folder. Apparently a lot of different tools have a similar issue. I copied aapt.exe over to platform-tools and everything seems to be working now.
I don't do much with mobile stuff and it's always on an occasional basis, but one of the first things I learned to do with an early phone was root to optimize and extend their abilities (I hung onto my QWERTY keyboard as long as I could) so being able to have these tools is useful.
Posting here in case it helps others (there are other posts on the web about the same issue) but also as a bookmark for myself when I need this info again. :-)