I suppose it's no help knowing that file type is an mpeg1 format. If you can convert in 'media encoder' convert to mpeg. I would also try renaming the extension of one of the smaller files to mpg, see if the players open it and, if they do, see if PPro will. Maybe After Effects will if you have that too.
Other people seem having trouble with that file extension have updated Quick Time.
Since it used to work, I would suspect a recent OS update, if you've done one, may be interfering. If you have a set point from when you know it worked you may wish to try that. HOWEVER I tried a set point reset once and it was a headache.