Test apps on iPhone

A new function in Xcode 7 is that I can test my apps on iOS devices without a paid developer account.
It works fine on my iPad air, but when I try it out with my iPhone 5s I'll always get some kind error message.

I have tried with multiple Apple-ID's, and i have restored my iPhone (from a backup)
I have tried with one of my friend's iPad to, and it worked fine.

I hope some of you can help me.

Update

The error message Xcode gives when trying to install an app on the iPhone.

Solution 1:

Turns out the issue was relating to the name of the device, I solved it by changing the device name from an unsupported, blank(), Unicode character.