So, I'm working on a site with Event Calendar and the 2.4 alpha, and I've just about got the admin part working correctly on my test server, but on the live server I'm getting a weird error when I try to create an event in a calendar:

The AJAX request:

Nothing is printed to the browser, but this was in the server error log:

I'm not really sure why this isn't an issue on my local dev server that (as far as I know) is using the same alpha build as the remote dev server, but I haven't been able to successfully trace through the code to figure out why this is triggering... any thoughts?

[Edit]
I have made a change to the module: I added support for an event to be in multiple calendars using a many-many relationship between CalendarEvents and Calendars and a CalendarHierarchy extension for the Calendar class, if that helps at all.