This is also shown in the PHPCalendar setup menu so I presume this is a bug with the application as opposed to anything I'm doing to display the calendar.
Yes it is repeatable... and there's an event in there right now. Have a look at December... and then January. If you have a look at the administrator, you'll get the overLib popup in January but with nothing in it.
I see this only happens with repeating events. I'll add this to the bug log and have the fix in the next release, Version 6.2.0. That version is in production now and there isn't a release date, but it is expected within two weeks.