Problem with simulating a Y-junction circulator in CST

Hi guys,

Not very familiar with CST.

I am trying to simulate a Y-junction circulator (20GHz). After building the model, it gave me an error " the propogation constant of mode 1 is zero, cannot continue"

Model seems ok to me (HFSS has no problem to run it). Does any friend here know this issue?

I attached the simulation file.

Thanks in advance.

Lee