That to me indicates couatl app is not shutting down upon exiting FSX causing the FSX.exe to hang and thus throwing up the BEX error. Please fix.
Sorry, but that's not possible. Couatl.exe is an external .exe and, by definition, it doesn't have ANY access to the FSX memory, not even in read (let alone writing and allocating it), Couatl.exe can ONLY allocate *its* own memory, which is under an entirely different and separate address space from FSX. It's not technically possible that Couatl, by itself, could crash FSX.
So, why your elimination process is misleading you that Couatl "must" be the case of this BEX error, which I understand "seems" logical, because you "fixed it" by not running Couatl ?
Because Couatl, by means of Simconnect commands, can ASK FSX to create some objects on its behalf, and of course it's what it does for most of the time.
Which means, if there's a problem with, for example, your video driver or video settings or video tweaks, such as an otherwise perfectly legal scenery objects would cause a crash in FSX BECAUSE of those video-related issues, you will see exactly this behavior: the crash will disappear if you don't run Couatl, so you are mislead thinking it was Couatl "causing" the crash, but in fact the crash was caused by a problem with your settings that caused an object that would normally work fine, to misbehave.
So, I'd start by resetting your FSX.CFG to default, by moving to another folder and have FSX create a new one. Only apply the HIGHMEMFIX tweak and nothing else.
If it's not a settings problem, maybe it's something more trivial, like a corrupted file on your system, if such file belongs to one of the objects created by Couatl, again you'll will mislead thinking Couatl is the problem when it isn't.
In this case, you might try uninstalling all FSDT products and reinstall them, to ensure all their files are ok.
I don't think there should be any need to add that no, of course, I don't have any crashes by FSX on exit, nor I ever had one, apart for the infamous uiautomationcore.dll error, which happens every time you play too much with FSX menus and it's a very well known problem.
Instead, if you search around for "FSX crash on exit BEX", you'll find plenty of different reports, dating since 2008, when GSX and Couatl weren't even planned...