Halo 1 does not run. Unknown file version (anti-cheat)

[Faisal Aslam]

I am unable to run Halo 1 now. It ran perfectly fine and I was able to complete the game till escape pod. After that, it kept loading like forever. I tried to close the game and rerun Halo 1 again and again. I only saw a message once about anti-cheat: unknown file version. There was also location of that file mentioned. However, that message did not come again even after several attempts of running Halo 1. This game worked perfectly fine before on this machine. Don't know what is the problem. I tried to run and check game files integrity but Steam says all is good. I have already played Halo Reach with no issues. Only this game has issues so far. I have not tried other games yet. Don't know if this is because of recent update. But that is a possibility.