My computer is running KEIL MDK4 and nRF51 SDK4. So far no problem with nRF51422QFAAC0.
Upgrading to Keil MDK5 would be a nightmare, so that I gave up after a few days of playing around on another computer.
Before I set up that computer with KEIL MDK4 and nRF51 SDK5, I thought by asking the question here may save me some time.
Thanks