I don't know how similar the the W126 tumblers are to the W201 tumblers, but I had that exact problem with my 190E last year. All of the locks worked fine but when I tried to open the driver's door with the key it would just turn back and forth without unlocking or locking the door. I removed the tumbler from the door and found that the small metal tab at the end of the tumbler that engages with the rest of the locking mechanism had snapped off. I was able to order a new tumbler from the MB dealer using my VIN.

I'm not sure of the similarity of the two tumblers, but the symptoms you described are identical to what I experienced.