I have looked on the forum and found one other post that has the same error, but this seems to be slightly different.
I am the webmaster for a siye using Advanced Guestbook V2.3.3. This was not an upgrade but a clean fresh download. The guestbook is at www.petit-illyria.eu.com
I am able to create guestbook entries without a problem, but the owners of the site are getting "One of the input fields does not seem to be valid." whenever they try.
Can anyone suggest what I should do to rectify this please? The other problem is that the site owners are in France and I am in England - so it's kind of difficult to visit their machine and look at what they are doing!