Calendar month view error
UPDATED RESPONSE...

We were finally able to get to the bottom of this and traced the issue down to your site likely having a DateTime field with the handle date, and this was apparently something Craft was choking on.

The next version of Calendar will fix this, but you can resolve this by renaming that DateTime field or patch this by looking for this code in calendar/services/Calendar_EventsService.php around line 100:

public function getEventList($criteria = null)
{
    if (!isset($criteria['allowedCalendarsOnly'])) {
        $criteria['allowedCalendarsOnly'] = false;
    }


And right after that, add this:

    if (isset($criteria['date'])) {
        unset($criteria['date']);
    }


Let me know if this resolves the issue for you. :)