Corsa Transmission Oil Temperature Sensor Fault

Sounds like the low oil level is a strong contender. I've seen similar symptoms when the gearbox oil is low, especially the grinding noises. The sensor is likely just reporting the overheating due to insufficient lubrication. Have you topped up the oil to see if the symptoms improve? Also, which error code are you seeing?

I haven't topped up the oil yet, as I wanted to get some opinions first. I can try that. As for the error code, I'll have to check again, but I think it stated something about an implausible signal from the transmission oil temperature sensor. If topping up the oil doesn't solve the issue, what else could it be?

An implausible signal indeed points towards overheating, which can very well be caused by low oil. If topping up the oil doesn't completely resolve the issue, it's possible the sensor itself has been damaged. There may also be internal damage to the gearbox, although that's less likely if you caught it early. Given the symptoms and mileage, I would suggest having a professional diagnose it to rule out any serious damage. At least they can read out the error code and give you a more detailed assessment.

I topped up the transmission oil, and while the grinding noises have subsided, the error light is still present. It seems a visit to the workshop is necessary. They found a faulty transmission oil temperature sensor and fixed it for 365€. Thanks again!