This has been already covered and it's a known issue: caused by the fact that some users want to patch FS9 EXE allowing for more than 2GB of RAM, we have TWO versions of our FS9 Addon Manager module, needed by the scenery.
One is compatible with >2GB, but it's not compatible with the original FS9 CD protection in the FS9.EXE
Another one is not compatible with the >2GB patch, but it's compatible with the original CD protection in the FS9.EXE
The installer will automatically check which FS9 version you have, and will install the correct version.
So, if you want to use >2GB, you need to patch a nocd FS9.EXE
After replacing the FS9.EXE, you need to Uninstall the Reinstall either the scenery, or the Stand-Alone Addon Manager.