That sucks! Yes, the kiosks is hit or miss with expired GC and I-751. Where kiosk operated in facial recognition mode - I was fine. Where it asked for a document - I had to see an agent.
Now I got 10 year GC, and updated expiration on GE portal, not sure if this us sufficient or have to see somebody at enrollment center.
I did not try updating expiration date based on extension letter before, maybe it was OK or maybe it wasn't.
The enrollment center avoids I-751 extension like the plague, I tried twice updating info in their system. Seems like the same is with new enrollment, though at least I could use GE lane with kiosks not working...