By the way, just to clarify from my previous comment, I dont beleive I have Toolkit unless I’m looking in the wrong place for it so still just get a black screen and then CTD. Would love it if someone could help me as literally spent the entire weekend trying things to fix this stupid game
Check your harddrive, just in case. In my case it was a bad harddrive. I had to reinstall on another disk. Was running MSFS on a regular harddrive since I’m waiting for my new pc build to arrive. After reinstalling to an ssd I had no problems what so ever. Crystal Disk is a free tool that let’s you inspect your harddrive. It came up with a warning showing several uncorrectable sectors on the affected hd.
By the way, also check the rest of your hardware. This might be hardware related. Do you have any problems in other games?
Hi Guys, I had a similar issue. In my case it was Bullguard security that was rendered incompatible following SU4. Once I uninstalled Bullguard I was able to launch the sim again. Hope this works for some of you
If you have not done any tinkering with your installattion,
Go to Settings , Apps
Uninstall MS FS
Go to MS Store or Steam and install it again.
It will download and install the initial 1.07 GB.
It wil not download the additional 170 GB (15 hours) if it is still intact.
If it is not detected, you will have to download it again.
Having pulled the last remaining bits of hair I’ve left + grinded the last of my teeth with a black screen at start up CTD ( which occured in the last few days , sim working flawless up to that point ) ,
I’m currently 9.04 GIB into a 85.33 GIB download .
I disabled Bullguard Security firewall + I disabled Microsoft Security firewall .
I will report back later when ( hopefully ) MSFS 2020 is reinstalled and working .
P.s. I uninstalled , reinstalled etc etc , threw salt over my shoulder etc over the last few days .
Sim installed , everything back up and running , mods back in community folder , no problems .
Bullguard Security + Microsoft defender firewall turned back on again , sim starting , no problems .
Great to have MSFS back again .
Haha! Aaand i am now downloading again… 
How on earth does one think of that. “Hey, let me dissable Toolkit. That could fix it.”
I hate and love this game so much now!
Thank you for the tip on disabling Seagate’s Toolkit. I couldn’t figure out what the problem was. I recently added a Seagate drive as a back-up and didn’t associate the problem with Toolkit. Like the guy before me said , “How on earth dies one think of that.” I sure am glad he did.
FS2020 - an adventure
On my side it was definitely Bull Guard which caused the CTD with “Exception code: 0xc0000409 on file : C:\WINDOWS\System32\ucrtbase.dll”.
I deinstalled Bull Guard → MSFS started successful.
I installed Bull Guard once more → MSFS crashed
I deinstalled Bull Guard, and rebooted my PC multiple times → MSFS started successful
Another hint: Disabling “Game Booster” in Bull Guard didn’t take affect. Just deinstalling whole Bull Guard made me happy, so I’m able to fly with MSFS again 
Have Bullguard myself . I found that turning off Bullguard firewall actually let me play MSFS again , and it was this causing my CTDs all along . I had no need to uninstall Bullguard .
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I don’t know that particular firewall, but I assume you could allow the FlightSimulator.exe through the firewall, and leave it turned on.
Its part of the C Runtime library.
This exception code Exception code: 0xc0000409, is a stack overflow. So something has blown the stack and the exception gets thrown and handled first time by the runtime. It doesn’t mean that urcrtbase.dll is the fault location, but whatever is using it hasn’t handled an exception.
I attached a debugger to my copy and found a number of access denied exceptions being thrown when attempting to get windows store login details from the registry. After that I got this stack overflow.
An additional information from me: I use MSFS 2020 since first version. My problem with Bullguard described above exist only since MSFS v1.16.2.0, before I never had problems with CTD!
Hi
I had a similar problem
my visual C ++ 2015 runtime had been removed by an uninstall of another product.
I reinstall the visual C ++ 2015 runtime and everything is back to normal
I detected it while looking at the event log application of WIndows 10
Same problem for me. No solution yet!
Same problem for me.
Did the update to WU5 and run fin until today.
In my case it also seems to be Bullguard. I disabled Firewall and now MSFS launches again.
Very strange because it run fine before.
same here!