Why has my S planner stopped syncing with Windows 8 calendar, how can I get it working again?

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AC Question

S planner has stopped syncing with Windows 8 calendar

I have a Samsung S4 mini and have been using s planner as my calendar app. I also have a Microsoft Surface Pro tablet and use the native calendar app on it. These two calendars have always synced (in both directions) until recently. Now there is no syncing in either direction. The only changes that I can think of were an Android update (to version 4.4.2) and I also uninstalled an old version of the Outlook app on the phone (a new version had already been installed so I had both on the phone for a while).

Is anyone aware of anything that might have changed to stop this from working?