Let's get the game back to the default settings next. You can do that temporarily and not lose any customization you've done.
With the game closed please rename your WTF folder to WTF.bak and INTERFACE folder to INTERFACE.bak. Delete the Cache folder entirely. These hold setting info and temporary cache files that will be recreated by the game the next time you launch. Renaming them allows you to rename them back later if this step doesn't resolve anything.
NOTE: Please also make sure to remove any and all addon auto-installers or auto-updaters you may have, as those can automatically reinstall addons for you, possibly causing further issues.
Now start the game up again, agree to the TOU and set your resolution. See if the issue is still there. If it is, none of this helped and you can put it back like it was by exiting, deleting the newly created WTF and Interface folders, and renaming the .bak ones back to their original names.
But if the issue is fixed, it means it was a setting or addon most likely.
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