Door locks on 1967 mustang, jammed, but fixed it, here's how
I had the same issue, I could open my door with the door handle, but as soon as I locked the door with the key, I couldn't unlock it or even open the door with the outside or inside handle. Luckily I had not installed the door panel yet, so I reach in and had to manually push the latch to open the door.
I figured it out after an hour or so.
There is a threaded rod coming from the door handle (push button), it is too short, it needs to be lengthened, and to lengthen it you need to un-thread it about 3 whole turns. Mine was rusted tight, so I sprayed it with WD40, but I didn't have the patience to wait for it to loosen, so I removed the rod, and got a few pliers and loosened it that way.
I re-installed the rod, and now everything works. Now I can open the door, and lock and unlock the door.