Modus start stop issues

I had a similar issue with my 2014 Opel Astra, although not a diesel. The symptoms you describe sound awfully familiar. The computer is likely detecting a voltage drop and disabling the start-stop to protect the starter motor. Have you had the battery tested under load?

Tested under load? No, I just checked the voltage with a multimeter and it seemed okay. But I guess that doesn't really tell the whole story, does it? Is there a specific way to test it properly? And what else could it be if it's not the battery?

Indeed. A simple voltage check is insufficient. A load test will reveal its ability to deliver current under strain, simulating the demand when the engine restarts. While other components could contribute, such as a faulty sensor, the discharged battery remains the prime suspect. I recommend a workshop visit to perform a thorough battery test and diagnostic check.

You were spot on. Took it to the shop, and they confirmed the battery was heavily discharged and defective. New battery later, everything's working fine. Cost me 290€, which is annoying but at least it's fixed now!