Portal Calendar Configuration Error

I'm working in a Power Apps Portal and need to create a calendar that uses data from Engagement Opportunity records. Let me emphasize that this is in a Portal and not an Outlook issue. I have created a list specifically for this calendar and mapped some, but not all, of the fields (which are not required, so I'm not thinking this is not a problem). https://docs.microsoft.com/en-us/power-apps/maker/portals/configure/list-calendar-view

While I can see a calendar, I cannot see the dates in the portal that exist on the Volunteer Management side. There is a vague red ribbon error above the calendar:  'There was a problem loading data for this calendar.' 

My research is showing that people just go for FullCalendar using liquid, but since it is preferrable that we use this out of the box option, I'm wondering if anyone here has some recent experience with this error?