"Catchable fatal error" when signing up for an event (since latest upgrade)

Since the latest update, when signing up for an event the attendee now gets this message on a plain page with the event name (linked to the event page) as a heading:

Catchable fatal error: Object of class WP_Error could not be converted to string in /home/coastcho/public_html/members/blog/wp-content/plugins/rsvpmaker/rsvpmaker-plugabble.php on line 1826

The sign up still works, the person is showing as an attendee.

Hopefully a quick fix?

I’m using a child theme of Twenty-Ten.

https://www.ads-software.com/plugins/rsvpmaker/