We found the problem, and it was due to your DLL.XML or EXE.XML missing this line:
<Launch.ManualLoad>False</Launch.ManualLoad>
This line can be True or False, but it's usually present. If you add it to your DLL.XML and/or EXE.XML, the installer will continue with no errors.
The top part of the EXE.XML file should look like this:
<?xml version="1.0" encoding="windows-1252"?>
<SimBase.Document Type="Launch" version="1,0">
<Descr>Launch</Descr>
<Filename>exe.xml</Filename>
<Disabled>False</Disabled>
<Launch.ManualLoad>False</Launch.ManualLoad>
The top part of the DLL.XML file should look like this:
<?xml version="1.0" encoding="Windows-1252"?>
<SimBase.Document Type="Launch" version="1,0">
<Descr>Launch</Descr>
<Filename>dll.xml</Filename>
<Disabled>False</Disabled>
<Launch.ManualLoad>False</Launch.ManualLoad>
Then, you see the various <Launch.Addon> sections for each module you have installed.
The XML files are located at:
%APPDATA%\Microsoft\FSX ( FSX or FSX:SE if you installed FSX Steam edition without having FSX already installed )
%APPDATA%\Microsoft\FSX-SE ( FSX or FSX:SE if you installed FSX Steam edition alongside FSX )
%APPDATA%\Lockheed Martin\Prepar3d
%APPDATA%\Lockheed Martin\Prepar3d V2
%APPDATA%\Lockheed Martin\Prepar3d V3
If you use P3D, depending on your version
Be sure you use a pure text editor to edit the files (Notepad it's fine), and check both the DLL.XML and the EXE.XML files. We are also updating all the installer to handle this case, but there's no need to download them again, if you just fix the XML files this way.