Try to disable all add-ons except the FSDT Addon Manager.
If those add-ons are properly made using the add-on.xml method, it should be extremely easy to temporarily disable all of them from the Options->Add-ons menu. Unfortunately, those that tends to cause such conflicts are also those that don't use the add-on.xml method, so you might need to either uninstall them, or manually edit your EXE.XML and DLL.XML file ( and there are 2 copies of them in %ProgramData% and %Appdata% ) to disable them temporarily.
If the problem get fixed this way, at least you would know is caused by a conflict between Simconnect version ( the sim being stuck using an older Simconnect, instead of the proper one we use for GSX, which is the P3D native )