If you actually have a look at the screenshots I attached, you will see that everything looks correct (except the 10.0 folder which hadn't been deleted at the time i took the screenshot).
It's *because* I've looked at the screenshot, that I told to remove the entry, because it WAS there!
You can see that coexistence mode is set to 0. I understand that if there is no FSX installation there shouldnt be a FSX:SE entry
Again, that's NOT what I've said and it's NOT what all the previous posts said. You SHOULD have an FSX:SE entry, which contains the Co-existence key (which should be 0). What you should NOT have, is the 10.0 folder INSIDE of it!!
but since there HAD been a FSX non steam installation, the entry is still there. What am I supposed to do if this is the case?
Remove the 10.0 folder, and be-sure the Co-existence is set to 0.
Besides in the link your posting you're only telling how the registries should look or should not look. I don't find an explanation on how to adequately change the registry entries in a proper way.
You must change them to become how they are supposed to be, which is exactly what I indicated in the previous posts. I really hope you do not expect to be given a tutorial on how to use REGEDIT: there are plenty of them online.
Please refrain from just linking the old topic, which I had read already before I contacted and assist me in finding a solution of my individual problem here.
That thread indicated everything you had to do. I can only repeat and confirm that, if you:
1) Fix the registry as explained
AND
2) reinstall the scenery.
It will surely work.