Probably both, IdK. Today it starts to flashing red and green, and when attaching the charger it turns into a solid green. No description of those alternative flashing between red and green can be found in the manual...
But all points to battery, bc this behavior happens when I come back from outside... in the morning, when the bike stays in the warmth overnight it starts the charge again.

Sorry for the lack of clarity!

@jonathanf2: I’m aware that the warning from the head unit is a bug; I just wanted to mention it.
@amalcal1: My battery dies suddenly, and I couldn’t shift.

Let me explain the issue in more detail:

It first started when I wanted to ride home, but the battery was dead. This was surprising because just a few hours earlier, I finished a ride without any problems.

I charged the battery, and the solid green lights appeared very quickly. That’s when I started checking with the app.
The app showed the battery at 100%, but when I disconnected and reconnected the app (without the charging cable), it dropped to about 8%.
When I reattached the cable, the solid green light came back quickly, and the app again showed 100%.

It got so bad that I could only shift gears once or twice before it stopped working, even though the charging cable had shown a solid green light.
So, I left the app closed, attached the cable after the rear derailleur went idle, and it started blinking green. However, whenever I opened the app, the green light turned solid again.

After charging (attach the charging cable when the derailleur is in idle mode) without opening the app, the green light stayed blinking and only turned solid after 2-3 hours.

This is why I suspect there might also be an issue with the app.