"This app does not have access to your calendars" in native Calendar app and in WeekCal app

I woke up this morning to repeated ping noises from my phone. I had to shut down the phone and restart it, to stop the sounds. When the phone restarted, both my calendar apps now come up with the error message above and are essentially unusable. This is a bit of a crisis!

I've checked suggestions re. screen time restrictions (didn't help) and have looked at the WeekCal entry in the Settings app and, oddly, there is no option there to select levels of access to my calendars, as suggested in some help articles (see screen shots).

I've also tried uninstalling WeekCal and re-installing it again. Didn't help. Not surprising, as I have the same issue in the native calendar app.

I'm running iOS 17.5.1

Any ideas?!