On my Note 2, I had both Google Calendar installed, in addition to S-Planner (which was pre-installed). What I found was that having both calendar apps installed was causing issues with Sync, as well as saving notification sounds.

Because of that, I un-installed the Google Calendar app and all was well. Not sure if the solution will solve the issue you're dealing with, but thought I'd throw that out there, in case it does solve the problem.

 

There is a long standing issue with syncing between S Planner and Google Calendar, there just isn't a workaround...and I don't think there ever will be unfortunately.