I had this and it was fraying of the stranded metal wire inside the plastic outer sheath (at the latch end inside the door). I tried to fix it, but in the end just replaced the pull cable. Worth popping the door card off to have a look and see what the situation is like inside the door/at the latch end.
When mine fully stuck it wouldn't let me open the door (even with the outside handle) as it was jamming the mechanism in the latch part way. I got lucky with a pair of pliers on the metal cable at the interior door handle end and freed it up so I could open the door and replace the cable.