Having a transcript of the English error message brought the solution, so thanks to all that helped out here and made me do the necessary stuff. On https://learn.usa.canon.com/resources/articles/2015/counterfeit_battery_protection.htmlp the reason and the implications of this message are explained.

What will happen if I chose either of the options? Here's a quote from that site explaining:

• Actual Canon-brand battery:
  No warning screen appears. Camera starts up normally and is ready to use.
• Camera cannot confirm full communication with battery:
  Warning screen appears. Here’s the sequence:
  Within 5 seconds: “Battery Communication Error. Does this battery display the Canon logo?”
  • NO > “Canon does not guarantee the performance or safety of this battery. Continue use?”
    If you select YES and press the SET button, the camera turns on normally. You’ve told the system this is a non-Canon branded battery and you accept any possible risk of a performance or safety issue. If you select NO, you’ve told the camera not to continue use with this battery and the camera will shut off. You can restart it by turning the camera’s main switch back to ON or pressing the On-Off button again, if you change your mind.
  • YES > “Battery may be counterfeit! Please call customer support. Shutting off for your safety.”
    A battery with a Canon logo (not a third party accessory, as discussed above) is one of two things: (1) a genuine Canon battery which cannot communicate with the camera possibly due to a defect or dirty battery contact or (2) a counterfeit Canon-branded battery, made to look like a genuine Canon battery but without the internal communication circuitry needed to complete the start up process with the select Canon camera(s).
    “OK” is your only option in this case; the camera will shut off to prevent potential damage to you and your property. You can turn it back on by repeating the start procedure.