Google calendar stopped syncing with my HTC one

[AC Question]

I have used my Google calendars succesfully withg my HTC one for a year syncing perfectly. However since June it has diplayed a sync error when I have tried to sync my calendar. My google drive on the same account is syncing perfectly! Please can you help?

 

[STARGATE]

Reboot the phone, log off the Google account, don't forget to back up before this, reboot again.
Log in to your Google account again, check if calendar is working again.