If your device is managed by your company using Mobile Device Management (MDM) there are some new restrictions in iOS12.
How to verify if your device has restrictions in place?
- Go to devive Settings > General > Profiles
- If you have an MDM profile installed you can verify if you have a restriction:
'Opening documents from managed to unmanaged apps not allowed'
If so, ask your system administrator to grant WeekCal access to managed calendars and contacts by adding WeekCal to managed apps in MDM profile.