Ajax Error on Master Auto-Preview

I have been having an intermittent fault which shows itself as an Ajax error, reporting a Server Error 500.

This occurs when viewing the Master display, and when generating the Preview, typically after switching to the Master view or pressing Save for the View. In the latter case the "View has been saved" message comes up, then the little circle spins for a while, then the Ajax error box comes up. I can view Previews on e.g. the Month display OK. Normal viewing of the Calendar page is OK.

This is correlated with a Drupal log message:
Location: http://mysite.org.uk/admin/structure/views/view/eventcalendar/preview/de...
Message: User Notice: 'calendar_plugin_style: A date argument is required when using the calendar style, but it is missing or is not using the default date.' in calendar_plugin_style->render() (line 232 of /home/sites/lalg.org.uk/public_html/dev/sites/all/modules/calendar/includes/calendar_plugin_style.inc).

I am also getting Server Log error messages:
[Wed Jan 30 03:02:01 2013] [error] [client 86.24.67.76] Premature end of script headers: index.php, referer: http://dev.lalg.org.uk/admin/structure/views/view/eventcalendar

BUT
Sometimes I get the Ajax Error without the Drupal error logged.
Sometimes I get the Drupal Error log without the Ajax error (though the little circle did spin for rather a long time)
The server Error Log entries are happening at about the same time, but don't exactly correlate with Drupal error log - I'm not sure if they are one-to-one with Ajax errors or not.

This is a development site and I am the only user on it at the time.

I don't know if any of this gives a clue as to what is going on.

Regards, Tony.