[TYPO3-english] Extension Builder :: "Error while saving!"

[mostwanted]

i get this error message after pressing "save":

--- quote ---
Error while saving!
--- /quote ---

only an empty folder typo3conf/ext/EXTENSIONKEY is created.

when i try to save the extension once again this error message is diplayed:

--- quote ---
Extension could not be saved: There is an existing piBased extension with this extension key. Please choose another extension key or rename the existing extension directory
--- /quote ---

my development enviroment:
Mac, MAMP, TYPO3 4.5, Extension Builder v.1.0 (via git / last commit: 2f7489c0c784d3958092426836cec5960d459056)