M Klasse losing power big time

Sounds like your alternator is failing. I had similar issues with my Mercedes a while back, though it wasn't a hybrid. Reduced power, flickering lights, and eventually, the car wouldn't start at all. First thing to check is the voltage drop across the battery terminals when the car is running. Should be around 14V. If it's lower, especially with the engine revved, your alternator is likely the culprit. A defective generator can trigger all those symptoms and store errors. Has anyone checked the charging system?

Voltage drop across the battery terminals? How exactly do I check that? And is alternator replacement a big job? I'm not super mechanically inclined, but I'm wondering if it's something I could tackle myself. Also, would a bad alternator cause the car won't start issue?

You'll need a multimeter to check the voltage. Set it to DC voltage, and connect the probes to the battery terminals while the car is running. Watch the reading. As for replacing the alternator, it's not the simplest job, especially on a hybrid. There can be high voltage components involved, so safety is paramount. And yes a bad alternator definitely can cause car won't start issues. Given the hybrid system, I'd strongly recommend taking it to a qualified workshop. They can properly diagnose the charging system and ensure the alternator replacement is done safely and correctly.

I'll get it to a shop. I appreciate the help, especially about the charging system and safety thing! They confirmed it was the generator, exactly like you said. Quoted 960€ for the repair.