Reach Anti Cheat Bug

On PC Steam, having a bug when I try to load in to a mission (the last one :() on legendary, having played it all the way through. It now goes to the loading screen, loads but boots me back to the menu, also have a load of pop ups about anti cheat.

Please help I want to finish Reach tonight!

https://youtu.be/fbgsvya0Ag0

Edit - had to verify files through steam