S60 Start Stop Problems

Sounds like the car is detecting a fault that prevents it from safely restarting. That's why the error and warning light. A weak battery is the most common cause. These systems are sensitive to voltage drops since the starter motor needs enough power to restart the engine quickly. Has the battery been replaced recently?

The battery hasn't been replaced recently, maybe it's the original. Can a bad battery really cause the engine to shut off even when driving, not just in Eco mode?

A critically weak battery can cause stalling while driving, though it's less common. If the voltage drops too low, the engine control unit might act up, or the car might shut down non-essential systems, or even the engine, to save power. Get the battery and start-stop system diagnosed. They can test the battery and check for other issues. My bad battery repair was 290€. Hopefully, it's the same!

I'll take it to a workshop for a diagnosis and will post the resolution.