iOS 8.1.1 devices "pending" after enrollment in Profile Manager

Setup:

OS X Yosemite with server 4.0

After installing the trust certificate and enrolling an iOS 8.1.1 client, I can see the specific device in Profile Manager. However the status of the device stays "Pending". It seems that the enrollment proces can't proceed.

When I enroll a device with iOS 7.1.1 there are no issues. Everything works fine!

Any suggestions?

Thx