Hi, I get it too...
Unless I've tried it before with DW still open... in which case it goes straight to that same error, which I must click twice again to clear.
I've found, on deleting that cache file, that there are two DW Config folders for some reason... only one of which deletes that file, btw... I am assuming that it isn't normal to have two config folders.
When I initially did that it went farther and partially displayed the ThemeIt screen, but there were problems similar to some folks above about onMouseUp, but I didn't write them down.
Windows XP Pro SP2 32 bit
Boxx 5404 Opteron 270(x2) 4GB