Found the problem, it was a bug in the Addon Manager related to support of legacy FS8 code, which is different (and not needed by most of our sceneries) in the FSX Steam Edition.
You only need to get the updated version of the bglmanx.dll file so, JUST run ANY FSDT installer (Stand-Alone Addon Manager, any scenery, GSX, etc.), no need to redownload anything, just run any one installer, to get the updated .DLL.
Sorry for the problem, it was a very old bit of code that we didn't use for years (it was pre-FSX), but still had to be handled with the new FSX version.