If you experience sync/data issues on Android, but your data is all fine online in your cloud service, you may want to try to reset the Android calendar storage.

THIS WILL DELETE ALL LOCAL CALENDAR DATA ON THE DEVICE! Make sure everything is synchronized/backed up!

To reset, go to Android settings - APPS - enable the option Show system apps/processes - Calendar Storage.

There select Storage and Clear storage.

Your cloud services should start syncing right away (if data/background settings permit).