S90 High Voltage System Error Resolved

That sounds like a serious issue! I had something similar happen with my old hybrid a few years back. Intermittent shutdowns and power reduction are never fun. Since it's flagging a high-voltage system error, it's likely related to the hybrid battery or the inverter. Have you noticed anything else unusual, like longer charging times (if it's a plug-in), strange noises, or any other warning lights?

Charging times seem normal. No strange noises or other warnings besides the high-voltage system error. Could a heavily discharged battery cause these symptoms even without other obvious signs?

Absolutely. A deeply discharged or failing battery can definitely cause those symptoms. The car's computer monitors battery health, and if it detects a significant drop in voltage or capacity, it can trigger that high-voltage system error and reduce power to protect the system. It's possible one or more battery cells are failing internally. I'd recommend getting it checked at a workshop with hybrid experience. It’s possible to measure and check each cell individually. In my experience it could also be a faulty sensor, but the garage will be able to diagnose that, too.

Thanks for the insight. Took it to the workshop, and it turned out the auxiliary battery was heavily discharged. They replaced it for 290€, and the problem is resolved.