I take it verifying files didn't help?

In Windows, do you have this problem if you start the Classic version of the game?

Are you shutting down all non-essential programs before starting the game, especially anti-virus, graphics utilities, or DirectX injectors, etc?

Try doing a clean boot and then test the game.
For Windows, click Start, or hit WinKey-R, type in msconfig and hit enter; in the General tab, click Selective Startup, uncheck Load startup items (if required) and leave Load system services and Use original boot configuration options checked. Next, click on the Services tab, check the box to Hide all Microsoft services, then click the Disable All button (maybe make a note of which are currently enabled/disabled), then click OK and reboot the computer.
Run msconfig again to switch back to the normal boot configuration.
For OSX, start/restart your Mac and hold the Shift key down as soon as the progress bar comes up in the startup window, releasing it after the desktop appears. Alternately, from the login window, hold the Shift key when you click the Log In button, and release it when you see the Dock.

Try creating a new Windows/OSX administrator user account, switch to that account and try starting the game from there.

In OSX, what file system are you using? I think case-insensitive APFS is default since High Sierra (HFS+ case-insensitive before that). You can check in Disk Utility (/Applications/Utilities).