Xsara diesel smoke and DPF issues

I had very similar symptoms with my Xsara. In my case, the exhaust back pressure sensor was indeed the culprit. Have you checked the sensor readings with a diagnostic tool?

Thanks for the info. I haven't checked the sensor readings directly. I'm not sure where it is located. Do you have any hints on where to find it?

The sensor is located within the exhaust system. Given your symptoms, including the failure of DPF regeneration, it's highly probable that this sensor is defective. I strongly suggest visiting a repair workshop for a professional diagnosis. It cost me around 245€ to replace it.

Thanks for the advice. I took it to a local mechanic, and you were right, it was the exhaust back pressure sensor. Replaced it, and problem solved!