I have been rendering from After Effects without a lot of messing around in Media Encoder settings, and until recently I get .mp4 animations that play in browsers, etc.
Recently AME has been producing mp4s that no browser will open; the available offered apps are QuickTime Player and Books and something called Handbrake. (Also, in QuckTime Player the color is extremely washed out; I think I have all Photoshop source files and PS and AE working spaces set to sRGB).
I may have done something when I was trying to adjust AE's color space; I don't know. But now even when I choose something designed for web, like a Facebook preset in AME, the rendered file will only open in QuickTime.
Sorry for the trouble. Could you share a screenshot of the export settings that you are choosing?
Do you check the playback on some other media player also?
Let us know. We'll get it checked.