İ’ve been trying to play Classic on windowed mode for the last few hours and I’ve been getting “Unable to start 3d Accelerators” error every time I tried. I redownloaded wow, deleted and updated my graphics card drivers.
https://eu.forums.blizzard.com/en/wow/t/cant-run-classic-in-windowed-mode/75144/5
Disable Discord’s background layout.
I completely disabled discord and chrome, which where the only other apps working besides from WoW.
You might be better off asking for assistance in Technical Support.
Did so, thanks 
Try this.
On the blizzard app, click the blizzard logo, go to options, then game options, then scroll down to classic and put in the additional command window “-windowed”
Had this issue trying to toggle vsync and some other settings.
FIX: Run wow.exe as administrator. Change settings and apply.
Now you can run it normally through Bnet
This is the link which helped me fix my problem with it.
NOTE: Ignore the mouse bit, I just edited the configuration file.
Run wow in win8 comp mode
Right click the exe, properties, run with win7 or win 8 instead of the version you have
That fixed all graphical errors for me
Including the one you have
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