White screen of death on IOS

Guess this question is asked several times already, but maybe some of you guys have a suggestion for that.
My ionic 2 app runs perfectly on android, when I build it on IOS - and that is the weird part- it SOMETIMES starts with the well known white screen. It happens in the emulator and on the device too, never in the browser.
So if you start it again, everything can be fine, and next or the next but one start again shows a white screen.
Nothing to be seen in the logs, only symptom I may see, DEVICE READY never fires in that case.
I read a lot about possible reasons for the white screen, but it always seems to be a permanent symptom.
Is there a way to get better diagnostics (already tried ionic run ios -l -c but no difference in log output).
Any ideas? Thanks a lot