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 device 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, contacts and reminders / tasks by adding WeekCal to managed apps in MDM profile.