Problem with FEKO UTD Simulation

Hi all,
I have a problem with Feko UTD simulation. The environment is very simple. I have a big box (2x2x6 meters) with inside an other box with only 2 faces and 4 pillar (see figure attached). The source is a spherical source. The frequency solve is 7 GHz and I calculate the near field in a plane with 21x21 point. The time for the simulation is very very slow (in one day I have finished only 2%). During the simulation i have some warning:

" WARNING 1188: Distance between two consecutive UTD ray points too small
See also message in the output file nimura5GHz.out
WARNING 1010: Assumptions of UTD not fulfilled
See also message in the output file nimura5GHz.out"

Could someone help me? thanks

Have you tried other FEKO methods? I'm guessing MoM was not an option. How about MLFMM? Can you use any symmetry planes in your problem to reduce the domain?
How about PO, will PO work?