In the past i've always used AE to do final prep work on my videos, and my final products have alwaus been exported with the H.264 output module, as .mp4 files.
However, recently, one of my projects ran into a bug where if I would attempt to add anything new to the timeline, AE would freeze, save, and crash. Someone online suggested resetting my AE settings to default, which I did. However, that didn't fix the problem, and in the end I just ended up copying all of the work to a new project file.
After I did this, my exports are now behaving a bit oddly. In the past, as I said, I would export as H.264, and it would create a single .mp4 file.
Now, it attempts to make a .m4v by default. If I export as a .m4v, it exports 2 different files: A m4v file, and an mpa file... the mpa file is audio only, and the m4v file doesn't open.
I tried simply exporting the video with a .mp4 file extension instead, but I ended up with the same outcome.
I have attempted to make my own custom settings, to see if that would work, but it didn't change anything unfortunately.
Odd note: With my custom settings, the first time around, I forget to check off the "export audio" box. The mp4 exported fine, and played with no problems, albeit with lack of audio of course. After checking off the "export audio" box, it began doing the split file thing again.