Without any system info and other critical details like your hardware acceleration settings we can't tell you much. It pretty much comes down to these two things 99% of the time. Either the update borked up your system CoDecs in which case you simply need to try and uninstall and reinstall everything or your hardware acceleration needs to be fixed by updating your graphics driver and configuring it suitably.
My apologies for the issue. From what I can understand, the .mp4 files you mentioned were working fine on the previous versions of After Effects but they don’t anymore in the current version. Is that correct? Are you facing this issue with all .mp4 files or only a few? Please share your system specifications (OS, CPU, GPU, RAM) as well as your video properties (format, codec, frame size, frame rate).