Oh yeah, I did it
In customer/home.tpl there was a typo in "{include file="meta.tpl"}" which prevented the file from being included. This meant common.js was not loaded on the registration page and from this script the element _getById could not be loaded. Simple.. but took me ages to figure out.
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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 