My S Planner has stopped syncing to my Google Calendar.

[Android Central Question]

Really need help here!

I have a Galaxy S6 Edge Plus. My S Planner has stopped syncing to my Google Calendar, and vice versa. The only calendar on my S Planner App is my Google Calendar/Gmail Calendar.

Very frustrating as I now essentially have two versions of the same calendar, and my handheld is filled with the wrong data. Anyone out there know what I can do to solve this issue?
P.S. All software is up to date...