Siri on Apple Watch 5 can't find appointments or create new ones
I have an iPhone XS on iOS 14.3 and an Apple Watch 5 Nike Edition on watchOS 7.2. Everything works fine - incl. Siri on the watch - with one exception: Siri on the watch can't work with Calendar on the watch.
Opening Calendar on the watch, I see all my appointments just fine. However, when asking Siri "what's my next appointment" it replies that there aren't any. If I ask Siri on the watch to add a new appointment, it replies "Sorry, I'm afraid I can't do that.".
The exact same commands work fine on the phone itself.
I've tried to factory reset the watch and restored from backup - no change.
I've tried to factory reset the watch and set it up as a new device - no change either.
However, during setup, there was some screen about Siri not being compatible with my region and language (Germany / English (UK)) and it being disabled because of that. But after the watch became usable, Siri was activated and happy to reply. Also, as mentioned, everything else regarding Siri on the watch works just fine. It answers questions about the weather, etc.
Is there anything else I could try?
Apple Watch Sport