Problems after iOS 17 upgrade

Calendars empty or appointments missing or sync errors after iOS 17 upgrade

Problem

One of the following problems can occur after the iOS 17 upgrade:

Calendars are empty
Appointments are missing
Sync errors

This can happen if you are using the calendar provider "iCloud on device".

Solution

Your data is still safe and available and this can easily be restored using the steps below.

Go to your iPhone settings
Now scroll all the way down and select the OneCalendar settings.
Now select "Calendars" option
Now select the option "Full Access"
Confirm by pressing "Allow Full Access"
Now go to OneCalendar -> Menu -> Sync to force synchronisation and that should resolve the problems.