This may help and save precious time.
Mediaelement did not switch properly between video formats: in FF I could hear audio but had a black screen - no video. Quite a common issue, I learned.
AddType video in htaccess did not solved the problem alone.
Apparently it's a server issue the causes a bug in the mediaelementplayer.css file: I changed .me-plugin position from absolute to static.
Don't know why, but works.