When attempting to delete a single occurrence of a recurring calendar event it looks like something is stuck somewhere where it keeps getting synced to devices (Outlook 2013 and Android 5.1.1 using the stock Exchange integration).

It doesn't show up in the SOGo web interface which is rather strange. Perhaps there is some command to get it to reindex calendar events or something?

The bug is rather weird as I attempted to delete 2 single occurrences of 2 separate events. They are both gone from the web SOGo interface, however one still shows up on my Outlook 2013 and a different one shows up on my Android device.

For testing I tried to remove the exchange account from my Android device and re-add it thinking maybe it was a syncing anomaly. I still see one recurring event instance that I deleted.

I can try to do some more testing/debugging on my end if you are interested. I tested once by adding a recurring event in Outlook and attempting to delete a single occurrence of that event and it looks like the same problem happened.