Thanks for your replies!
@HDLI: 1.) so i should go back to dependent/independent boundaries? 2.) I assign the current density terminal to the outer radius of the coil. 
And the current density to the complete coil with cylindrical coordinates: 
Do I have to assign all 3 current parameters? What would be typical values? Since the current density on the inner radius is higher than the outside, how do I cope with that?
@Reshmi Raghavan: I added a small airgap between coil and core, but still the simulation failed.
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