CST waveguide port problem

I am a beginner user of the CST and I am simulating SRR structures. I want the waveguide ports as close as possible to the SRR. When I put the waveguide ports at the end of the SRR structure it gives me the folloving error.

"Waveguide for port number 1 is too short. The waveguide must be homogeneous for at least there mesh planes along propagation direction"

How can I solve this problem and get very close ports to the SRR?
Thanks

Hi,
you must little change your structure or increase mesh cells. Can you show me your structure?

Hi,
I attached the CST file. For this structure, I want to get a unit cell with 2.5mmx2.5mmx2.5mm dimensions. However, when I applied this condition, SRR touches to the ports. How can I solve this problem without changing the port distances and SRR dimensions?
Thanks