Ok I didan additional thing and that is to delete FSX.CFG file. This if corrupted can also cause problems. If add-ons have been installed it is a good idea to delete and let FSX rebuild. This has got it working now even with PMDG models.
Spoke too soon as a new bug it seems has appeared. Again with the PMDG aircraft there is a problem with the Airflow. When I fly these aircraft without the FSRealWx loaded or running in the background they work fine (tested) but when I load the weather and over a certain altitude the thrust and airspeed indicators go crazy. It will not register above Mack 4.9 and I cannot switch between Airspeed and Mach will not work. Not sure what part of the weather is interfering but when flying with normal MS FSX weather the problem does not occur. As per my previous test I did a shorth flight Where I did not go above FL200. This problem it seems occurs when you start to cross FL210.
That is probably due to the wind. FSX is not that good at interpolating the wind. You can set "Smooth Wind" via FSUIPC or activate "Direct Wind Control" in FSrealWX. There is also a difference in the "Weather Mode", "Direct" or "Weather File".
If you find an error, then please give me exactly about your system. Which sim (FSX / P3D / Xplane). Which connection (FSUIPC / SimConnect / FSrealWX-Bridge). Which OS.