Another update: I have also tried to launch with all peripihials except the mouse and keyboard disconnected, but no luck. From the troubleshooting page on Zendesk, I ran Windows Memory Diagnostics and it is reporting some errors. I tried turning off XMP and lowering the memory clock, but the sim still CTDs at the exact same spot. I have ordered 2 new sticks of ram which will arrive sometime tomorrow. Hopefully that will finally resolve this. I will report back.

The above will repair or isolate logical errors in the file system and also physical errors on the drives.

2 hours ago, turnandbank said:

I saw your thread on that. Hope you can get it resolved too. This is certainly discouraging.

I've had no success at all doing anything. 

6 hours ago, Fielder said:

The above will repair or isolate logical errors in the file system and also physical errors on the drives.

Thanks for that. I already ran those commands yesterday. Some corrupt files were fixed by sfc /scannow. I ran them all again today and they all came up error free so it seems that the issue is not being caused by the OS.

21 hours ago, turnandbank said:

Pretty frustrated at this point as I am running out of ideas. Any other suggestions are appreciated, but I do not plan on reinstalling Windows, as it would take to long to reinstall and configure all the apps I use for my work.

https://www.youtube.com/watch?v=ldqFdcGL1X0

since  by now  you  have  tried  everything  else,   is  to  uninstall/ reinstall  msfs unless  some  one  else   has  a better  solution

Edited May 26, 20242 yr by pete_auau

Problem resolved. After replacing my 2 sticks of RAM that Windows Memory Diagnotics was showing errors for, MSFS would still CTD at the same spot during the loading of a flight.

However, after uninstalling the sim from the Xbox app, then carefully deleting and removing all traces of it from my PC including cleaning the registry, and then reinstalling from the Xbox app, it finally works again, including after I restored all of my mods and addons. 

Even though MSFS was working fine the last time I used it prior to SU15, I think the faulty RAM must have corrupted some files during the update and again during the first two full reinstalls. If that was the case, the Xbox Verify and Repair function did not help either.

Anyway, just wanted to let everyone know the outcome and to thank those who offered advice. Hopefully my experience helps someone else. Now back to flying!