V60 key battery low

Sounds likely. I had a similar issue with my previous car, although it was a different brand. Usually, those symptoms point directly to a dying key fob battery. Have you tried the spare key to confirm?

Good point. The spare key works fine. So, I guess it’s confirmed: time for a new battery. Is there anything I should watch out for when replacing it?

If the spare key works perfectly, it's almost certainly the battery in the other key. Replacing it should be straightforward, but if you are unsure how to do it, maybe it's better to visit a workshop nearby to not damage the key. They can replace the battery for you without any risk. Alternatively, if you are confident doing it yourself, check if you need to re-pair the key with the car afterwards.

Thanks for the confirmation. A new battery (CR2032) did the trick, cost me 37€. All working now!