For anybody with similar problems. I found the culprit. It was the greek letter "µ", which was contained in the preshared key. Apparently IOS and the HP MSM APs use different codepages to map this greek letter. (It does work with any Windows PC/Laptop/Tablet/Phone and with Linux without problems.)
I exchanged this letter against another - non greek - letter and everything started to work smoothly.
Interestingly my other special symbols did not make any problem.
So problem solved.