OK, it's beginning to look like this is an obscure bug in PHP 4.4.x. I spent about 10 hours over the weekend trying to track this bug on your site, and our code is solid. For some reason the variable we are setting with the element HTML ID (the 'fabrik_formdata_X___field_name' thing) is getting set to NULL before it is used to build the form.
The same code works just fine on all other versions of PHP, 4.x and 5.x. We've applied a band-aid to the latest SVN rev, which should make this work for you.
If you have the time, it would be really useful if you could upgrade your local PC to PHP5, and see if the problem goes away WITHOUT upgrading Fabrik with the new patch.
-- hugh