Constraint Error syncing with google calendar

I am running One Calendar on Oppo. I never had any trouble before and suddenly I started to get sync errors. I had added another category in my google calendar that was not updating in OneCalendar. So, I have uninstalled the app and re-installed it. When I try to add my google calendar, it does load up and does show me the new category, but as soon as I try to add google calendar, I do get an error message: "Constraint".