Mercedes Citan Automatic Gearbox Problem

It could be. Automatic gearbox issues are tricky. When was the last time the hydraulic fluid was checked or replaced? Are there any other electrical issues? Low voltage can mess with the transmission control unit.

I haven't had the oil changed, I think, and the car battery was flat a few weeks ago. What do you mean by undervoltage? How could that affect the transmission performance?

Undervoltage can cause problems with modern car electronics. The transmission relies on precise electrical signals. If the voltage is unstable, it can cause the shifting problems and put the transmission into limp mode. If the battery was flat, it's possible it has damaged the transmission. I'd suggest a proper diagnosis. Oil replacement is likely required.

I took it to a workshop and they confirmed it was the oil and undervoltage. They did an oil replacement and it's running smoothly again. Cost me 240€. Thanks for the help.