From various flightsim related forums I read, it is highly recommended to install FSX, P3D, etc... OUTSIDE of the windows standard folders like Program Files (x86) or Program Files, as with the latest Windows versions, the OS is controlling user's rights on read, write, or amend operations more than previous versions (as far as I could understand...).
My P3D is installed straight at the root of my F drive, straight in a "Prepar3D v4" folder (to get path to the simulator as short as possible).
I installed all my flightsimming related softwares outside of the Windows Program Files (x86 or not), and further run them as administrator (but I am the sole user of my PC, and assume I know what I am doing....)
So, when/if you have to reinstall P3D, this could be a possible alternative...