Thanks for the tip with B4XDialog, I think that is, of course, a better way but...
It had nothing to do with my problem. It is like you has a problem with your car, a Mercedes and the dealer tell you: Why are you not driving a VW?
This is only a side remark because I was able to solve my problem by myself. I removed Xcode and the mac-builder and deleted all related subfolders, rebooted my mac mini and installed everything again and it works. I also disabled the automatic update function, maybe that was the reason? No idea, but it works now.