I have had this same issue intermittently. I usually have to lock/unlock the car a few times to get the fuel door to open, real annoying at a gas station...

When this doesnt work, I usually pull up on the red actuator part and it unlocks. Seems that sometimes it moves up a bit when you unlock car but doesnt move up enough to clear the latch.

If this still doesn't work I have taken a plastic putty knife and wedged down the top left corner of the door to clear the latch.

Hopefully that atleast helps you get it open... Not sure about how to fix long term.

Good luck.

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1995 W124 M104 E320 Wagon. 175k Miles and Counting