The custom form could not be opened

This is weird. We have never used any custom forms for anything,
anywhere in Outlook.

The receptionist goes into a public folder called Company Contacts.
She selects a number of contacts and chooses to send an email. First
she gets a warning some names don't have addresses. Then, she gets
the error "The custom form could not be opened, using the default form
instead" and that's it. No email with a list of addresses as one
would expect.

Any ideas? We do not use custom forms for anything.

Thanks!