I had a similar problem recently after downliading an add on. In my case I managed to remove the add on files running the 'regedit' process:
'Windows +r'
in the box type 'regedit' & 'enter'
click on the Windows C drive
Select 'File' & click on 'Extract'
Make sure you have still selected the C drive & Type: Config_backup & 'enter'
'Control'+f key 'Enter'
Type the name of the software you wish to remove 'Enter'
The first unwanted file will be highlighted
Delete the file
to find subsequent files press F3 & repeat until the regedit process says 'no more files found'
Close & restart computer & start FSX.
With the problem siftware removed FSX should load ok. It did in my case so I hope you have the same success. Best wishes.