Seat Leon gearbox issues

Whoa, that sounds rough! A failing torque converter can definitely cause those symptoms. The engine surge especially points towards the lock-up clutch malfunctioning. Does the grinding noise happen in all gears or just some? Also, did the check engine light come on before or after you noticed the other issues?

The grinding is mostly noticeable between 2nd and 3rd gear. The check engine light and transmission indicator light lit up at the same time after I felt a big shudder and loss of power. Could it be anything else besides the torque converter?

Sounds like a classic torque converter failure, especially with the grinding between 2nd and 3rd and the sudden shudder. That lock-up clutch in the converter is probably toast. Given all the symptoms and the error code, it’s highly likely the automatic transmission converter is defective. I'd recommend taking it to a reputable transmission specialist to confirm and get it sorted. It's not a cheap fix, unfortunately.

Took it to a transmission specialist and you were spot on. Torque converter was indeed the culprit. Ouch, 2120€ later, it's fixed, but at least it's running smoothly again. Thanks for the help!