Mini One Diesel Trouble

Sounds like you've done some solid troubleshooting already. A faulty fuel filter heater can definitely cause those symptoms, especially with a diesel engine. Is the error code you're seeing specifically pointing to the heater circuit? Have you checked the fuel filter itself for any signs of blockage or excessive dirt? When was the last time you had the filter replacement done?

Okay, with the error code specifically mentioning the heater and the mileage being that high without a confirmed filter replacement, it really does sound like a combination of things. A clogged filter puts extra strain on the heater, potentially damaging the connector. I'd recommend getting the fuel filter replaced and having the heater wiring properly inspected at a workshop. They'll be able to diagnose the issue accurately. Continuing to drive it with this issue could lead to engine stalling.

The error code does point directly to the fuel filter heater circuit. I haven't physically inspected the filter itself yet, but that's a good point. I don't have any record of a filter replacement in the service history since I bought the car, and it might be overdue anyway. Could a clogged filter amplify the heater issue or cause similar symptoms on its own?

Just wanted to give an update. Took the Mini to the workshop. Turns out it was indeed the fuel filter heater cable. They replaced the defective connector for 95€. The car runs like new now, the rough idle is gone, and I'm hoping the fuel efficiency will improve. Thanks a lot for your help!