I am not from Laminar Research; just a flight simmer from down under using a windows based system.
The full version X-Plane is exactly the same as the demo version except:-
- the demo version restricts you to a time limit of 10 minutes and has a fixed location in Seattle with very little scenery (mostly water);
- the current demo version is XP11.36 whilst if you have ticked the "beta version" box when converting from demo to full version then that will be the significant difference.
Read the following link found at https://forums.x-plane.org/index.php?/forums/topic/160879-thinking-of-downloading-a-x-plane-beta-version/ regarding the use of Beta versions.
I am also of the opinion that I have seen or read that the latest version of the Mac OS will not work or run with the current XP11.40 Beta version.
The downloaded demo version is XP11.36 (the last official X-Plane release) and your system works/worked well and when you installed the full version you have ticked a box to allow the Beta version to be included in the download. Hence your current problem
I have sent a private message to one of my contacts within Laminar to ascertain the current status of XP11.40 and if the problem of Mac OS compatibility has been fixed.
In the meantime as the demo version allowed you to run X-Plane I suggest you delete your current complete X-Plane installation to ensure version 11.40 has been totally removed and perform a clean installation making sure you do not allow the Beta version to be installed. In other words do not tick the box when asked if you want the beta version update installed.