Actually the registration button has always worked fine until we moved servers. I also changed the way the page loads up. The top part of the website was moved all the way down in the source code to let the search engines read the important content first. That's the only thing that has changed, no changes were made to the registration routine

But I suspect that now that the top of the page is loaded last this is causing JavaScript errors because there was another script that gave an error. The form was custimised, correct, but it worked fine before. So strange and difficult to figure out what's triggering this error. Thanks for the help!
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www.qualitylamps.eu
X-Cart Gold 4.1.8, PHP 5.2.6, MySQL 5.0.51a, Apache 2.2.9
www.vervangjelamp.nl
X-Cart Gold 4.2.0, PHP 5.2.6, MySQL 5.0.37-standard, Apache/2
Sorry for the weird user name, I registered it by mistake