CST Human head simulation with microstrip antenna

Hi all,
I've been trying to simulate a tiny microstrip antenna inside a human head model (layered sphere) and got these error messages:
1. Could not write PEC into file cstproject\microstrip and head\Result\Model.fpf
2. Error in calculating solver matrix.
I used the exact same model on the previous simulation, but with bigger size antenna, and it worked perfectly. I am wondering why it is not working using exact same setting but just smaller size antenna.
Here is my spec:
f = 0-20GHz
antenna size = 1.47 x 0.98 x 0.5 mm
Solver = Transient Solver
Boundary = Open (add space)
Please someone help me :)
Hans

Hello,
Do you have resolved this problem?I have the same one as you. Please help me and give me some advices.Thank you!