Ionic3 build Error - upgrading from ionic2 to ioni3

Ok, I am officially out of ideas. Something is breaking the build for your project, but I have no idea what it is and how you could debug this further.

1. ionic cordova run android --verbose works without problems, correct?
2. The only remaining hint is the almost useless error message: Search for "fixed" in your code, comment that stuff out, try again.