[Flow] Adding an interface leads to "Execution of subprocess failed with exit code 255 and no output"

[Christian Eßl]

I think I found the source of the problem. After I set "display_errors" in the php.ini to "On" (PHP 5.4.17 by the way) I get meaningful errors from TYPO3 Flow. 
I don't know if it's supposed to be set to "On" directly in the php.ini or TYPO3 Flow is supposed to do the setting automatically, when in Development Mode, but since I am anyway using the server just for testing and development, I will leave it for now.