I'm attempting to model a simple grounded coplanar waveguide transmission line and found that when I try to calculate the port modes at the center frequency (Right click port > Object information > Calculate), the calculation aborts with the following message:

Port mode solver setup failed for port 1

No other information is reported for port 1. Note that I've configured port 1 as a waveguide port with 3 modes and dimensions somewhat larger than the recommended in the CST help section for grounded coplanar waveguide (3w above trace, h below trace, and a width of 8w). However, if I attempt to do the same thing for port 2, with identical dimensions and configuration, the port solver also fails but with a message:

Calculated mode propagation constant(s) can not be resolved by mesh step. Please consider refining the mesh in the port region for port 2.

After bumping up the number of mesh cells per wavelength near to the structure from the default 20 to 50, I repeated the port mode calculation. It again failed as above for port 1, but for port 2 the calculation did complete with the following results:

None of these modes are propagating, nor do they match any of the QTEM modes displayed in the 2D port mode results after I run the full time domain simulation. 

For reference, here are the relevant model and simulation parameters:

Center trace width (w): 15 mil

Top ground gap width (g): 12 mil

Bottom ground gap height (h): 8 mil

Transmission line length (l): 1 mm

Dielectric: Rogers 4003C (lossy)

Conductor material: PEC

Simulation frequency range: 8-12 GHz

Solver: Time Domain

Mesh type: Hexahedral

Background: Vacuum

Bounding box: Open boundaries (add space)