Took me a while to realise why I had an empty calendar...
I have a content type with several date fields on - a user can choose which one to populate depending on whether it's a regular date/time, an all day date, a date range or a repeating date. All the date fields are added to the fullcalendar view and the events all show as expected. Unless you have a node where all the date fields have been left empty. This somehow breaks the calendar completely so no events are displayed (no errors logged either).
Closed: outdated
1.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I've volunteered to help clean up the FullCalendar issue queue. As part of that effort I'm closing issues open for unsupported versions of the module as "Closed (outdated)".
If this issue is still relevant for version 3.x of FullCalendar, then feel free to re-open it and update the issue version number.