Hey there, my door stopped opening so I popped it open at a gas station with a strip of polyester cloth from a t-shirt. If anyone finds this post, this may be useful in an emergency.
Right this moment the fuel door actuator is being replaced at the dealership for this exact issue. It was sporadic at first but became permanently shut eventually.
If you decide to follow this advice be warned that I am not responsible for any damage you may cause to your beautiful K5
How to open fuel door in an emergency (danger! this could damage your fuel door, use only in emergencies
1. Use a strip of cloth about 1" wide
2. Gently slide the cloth behind the fuel door from the right side until about midway or the cloth stops near the middle.
3. Use both ends of the cloth together to pull (don't yank) the door open with gradually increasing force
4. Take a break if it doesn't work and reposition the cloth vertically if it comes to an angle
5. The door will wobble up and down a bit, this didn't seem to cause any damage f
Here's a low quality screen grab I have of the process, note the cloth behind the door: