I remembered that I have helped you modified a similar structure, only difference is the location of the 4 waveports, In you model, no ground plane for coupler and no groud reference plane for your coaxial feed. The most reasonable cause of meshing error is the coupler part, you can check, no material assigned to them. I have tried to assign them to PEC, but I think you may need to consider the metal loss and etc,. the best way is to redraw this part, assign them material prtoperty and boonlean them correctly.
Best Regards,