Is the shellstyle in the folder you opened the theme in for the OS you're testing in? The only difference in testing in the beta is that it copies the shell folder from the directory you loaded the theme in to the testing directory instead of Aero's shell folder.
EDIT:
I cleaned the C:\ProgramData\Windows Style Builder\Testing folder and rebooted. It seemed to fix the issue. No clue how it happend though.
Here is whats happening when testing with many themes in different locations. All produce the same error. Keep in mind that the Aero default theme is untouched. I only started having this issue a few builds back.
http://youtu.be/qrTfzhxfgA0