Basically, as the title says. I’m trying to change my game from windowed fullscreen to windowed at settings. When I click on “apply” it changes from fullscreen to window several times, and shows me this error written above (and closes up the game completely).
I updated my GPU drivers, but it didn’t help. Any suggestions?
Good morning,
The first step would be do disable all possible Overlays that you might have like Overwolf, Discord, GeForce experience etc. Restart your PC and try again.
Kind regards
What’s your opinion?
Hi! I disabled the only overlay I had (GeForce Experience) and it didn’t help.
I also tried one of the fixes I found on forums - To “Disable full-screen optimisations” at wow classic .exe file properties, and that didn’t help as well 
Hello again,
Although it is being looked into already, you can try the additional steps from this article here.
Kind regards
What’s your opinion?
Hello Déathwish,
I can see that you are back online, so I assume that, at least for the moment, your issue sorted.
Kind regards
What’s your opinion?
OK. I did something very random and it fixed the issue.
In classic wow.exe properties I checked “Run this program in compatibility mode for Windows 8” and now I can turn on windowed mode with no issues.
Hello again,
Thanks a million for letting me know. This might indeed be helpful for others as well.
Kind regards
What’s your opinion?
That worked for me as well…thanks a lot!!! 
Glad I could help! 
OMG, you are a saint! thank you so much!! <3
I was able to fix the problem as follows
Firstly right click on the launch wow icon , go onto properties - run as administrator - and or / enable compatibility mode.
Secondly - and this is VERY Important - DISABLE DISCORD COMPLETELY -
as long as discord is running - I kept getting unable to start 3D accelerator
Something must have happend in the process of removing layer upon layer of features from the BfA client to the classic client, during which this issue arose.
I tried opening BfA and had no issues changing all of the graphic settings windowed, resolution size, testing changing all of the graphic settings and alternatives.
Not ONE crash, while there is 5 or 6 ways to trigger them in the classic client.
Been in queue a bit too long today to reset it now, but I will try the solutions found above tomorrow.