"Add Form" is broken in MailPress 5.1.1

[Graham Stoney]

I'm having the problem reported in the following thread, where the Add
Form button in MailPress 5.1.1 clears the form creation form without
adding a form to the list on the right hand side:
http://groups.google.com/group/mailpress/browse_thread/thread/9ccabedb1f952196/71be26e3c4cc9c40

The response to the POST is:
<?xml version='1.0' standalone='yes'?><wp_ajax><response action='add-
form_0'><form id='0' position='1'><response_data><![CDATA[]]></
response_data><supplemental></supplemental></form></response></
wp_ajax>

It only happens on some of my sites, but when it hits you can't create
a form. I've tried disabling all other plugins, and that doesn't help.
I'm using a fairly vanilla TwentyTen-based WordPress theme. I looked
for Javascript errors in Firebug, but can't see any error messages
when clicking Add Form. How can we track this problem down?

Thanks,
Graham

PS: Please don't reply via email; use http://buildyourblog.net/contact
to contact me privately.