Got this reply from Zendesk. It might help as I disabled all non MS apps and Flight Sim worked.
To resolve crashes due to ucrtbase.dll, please do the following:
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In the Windows Search bar, type “Task Manager”
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Select Open
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Go to the Start-up tab
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**Disable all non-Microsoft applications and services ***
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Reboot the computer
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Try launching Microsoft Flight Simulator again
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Apps that are known to cause Microsoft Flight Simulator to crash:
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Nahimic service
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Asus Sonic Studio 2
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Windows Audio Service
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Seagate Toolkit
If the sim stops crashing
To find out which application causes the sim to crash on start-up:
- Enable one non-Microsoft application
- Reboot the computer
- Launch Microsoft Flight Simulator
- Enable a second non-Microsoft application
- Reboot the computer
- Launch Microsoft Flight Simulator
Repeat this process until you find the application that is causing the crash. You can then disable this application or stop the process in the task manager before launching Microsoft Flight Simulator.
If the sim keeps crashing after disabling all services & applications, please do the following:
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When launching Microsoft Flight Simulator Type “Flight Simulator” in the Windows search bar and select Run as administrator instead of launching the sim from the Microsoft Store or the tray.
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Disable Malwarebytes or any similar security program that may run in the background.
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Uninstall Visual C++ and (re)install the latest version of Visual C++ (Visual Studio 2015, 2017 and 2019) then reboot your computer. To do so, follow this link: https://support.microsoft.com/en-us/help/2977003/the-latest-supported-visual-c-downloads
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Finally, ensure that Direct X is up to date: How to install the latest version of DirectX - Microsoft Support