Database error with submitting form

Hi,

I have a form on my blog, but some people get the following message while trying to submit the form: “Database error occured. Please try again.” I don’t know what to tell these people why it doesn’t work with them. Because not everyone has this problem while submitting the form.

Any ideas about what the problem could be?

Thanks in advance!

The page I need help with: [log in to see the link]