This is an old problem and you would think that Adobe would have a fix for it by now. But NOOOOO...... That's just common sense and we can't have that!
Alright the first thing to try is clearing all the media cache files and doing a full reboot. If that doesn't work then try holding down the alt key while starting premier and selecting restore default settings. Now should those 2 options still not work then select all the media in the time line edit zone and try exporting again.
Unfortunately, it looks like you're fucked if none of the above work.