What is CalNCService and why does it run hot in macOS Sierra?

Since the upgrade to macOS Sierra I occasionally (about twice a day?) see a service CalNCService running hot (i.e. on 100% CPU). So going from the name, this seems to be some Calendar Service for Notification Center. One can kill the service or restart the computer, but it just comes back. Usually, after a few hours or so it goes back to normal.

What is this deamon doing? How can I debug this issue?

I already found that the problem is connected to syncing reminders. Disabling reminder sync under Internet Accounts makes the service go back to normal, enabling it again makes it go back to 100%. This seems independent of the server. I have a bunch of recurring reminders and first had this problem with my account on fruux.com. Then I migrated all to iCloud and now its the same there.

Further remarks

• I tried clearing out the reminders list (it had 3600 done reminders, now it has only abour 20 left). This did not change anything.
• I tried disabling notifications and anything with calender in Notification center. This did not change anything.
• In the log console I find this when calNCService starts up:

warning: dynamic accessors failed to find @property implementation for 'serialNumber' for entity ABCDAddressBookSource while resolving selector 'serialNumber' on class 'ABCDAddressBookSource'. Did you remember to declare it @dynamic or @synthesized in the @implementation ?

This is a crosspost from http://apple.stackexchange.com/questions/256908/what-is-calncservice-and-why-doe s-it-run-hot-in-macos-sierra