The following hotfix solves several issues, and expecially the export function for P3Dv4!
Note: this is not a full installer, but a replacement of the original PFPX.exe file. Unzip the file and place it into the following directory:
c:\aerosoft\Professional Flight Planner X\
Important: PFPX requires 'Microsoft Visual C++ Redistributable for Visual Studio 2015 x86'.
PFPX Hotfix 1.28.9i.