Calendar not syncing to Outlook 2013 via EAS but other apps work fine

I’m using Windows 8.1 which features native Windows 8 Mail and Calendar apps. These apps allow you to use an EAS (Exchange Active Sync) account and Mail-in-a-Box works perfect with it - I add an appointment in OwnCloud and within 2 seconds it appears in the Calendar app. When using Outlook 2013 however, mail syncs straight away but not Calendar. I have set up the account in Outlook as EAS but not sure what’s the issue.

FYI, if I add an appointment in Outlook 2013, at the bottom it shows WAITING FOR SERVER RESPONSE... and the appointment disappears from Outlook after 3 minutes.

Also confirmed the issue is happening from multiple PC’s. Again, the native Calendar apps that use EAS are all fine, as well as iOS and Android.