I deleted that file and actually had to uninstall the FlyInsideFSX program as FSX continued to use this config file even if I was not running the program. Only after uninstalling it, FSX reverted to the original fsx.cfg file and now everything works ! It was not a FSDT related problem after all.
Yes, that explains it.
I still don't know how FSX knows to use a different .cfg file which is not even named fsx.cfg. Anyway that is a different problem for me to research.
I think it's related to how FlyInside works:
http://forum.aerosoft.com/index.php?/topic/119439-file-not-found-when-trying-to-use-flyinsides-cfg/It launches FSX itself, and the first time it started, it creates the fsx_flyinside.cfg, which is *derived* from the FSX.CFG which was valid at that time and, since it starts FSX itself, the sim will use the fsx_flyinside.cfg, which is the name of the main .exe process.
In any case, I can see issues if any of your addons makes changes to the FSX.CFG *after* you installed FlyInside, which in this case would prevent it from working, like adding its own Simobjects paths, which is something many addons do ( SODE, UT2, GSX, etc. ).
A proper way to handle this, would by that FlyInside should compare the last modification date of the "real" FSX.CFG against its own and, if the FSX.CFG it's newer, rebuild the fsx_flyinside.cfg file again, taking the new settings from the FSX.CFG and apply its own changes.