[Erwin]

I suspect it may be your PHP version. What version does your server use?

It probably is the "if (!isset($action))" command that your site is having a problem with - since it doesn't recognise this line, it automatically goes to "action=send" and because you haven't filled out any fields yet, you go to the error message as not all fields are entered.

This can be avoided by dividing the contact.php to 2 files, and not use the action variable... but that doesn't really make sense, since other vb files surely use the same variable too.