Hello David @Eim2,
Please check the following in that order:
- if you are using SEO URLs, check if you added the appropiate SEU URL support and check all modifications.
- check the modifications in tep_href_link function in includes/funktions/html_output.php
- check the modifications in tep_redirect function in includes/functions/general.php
- check the modifications in includes/configure.php
If you can't find the error, please send me your store URL in a private message and I'll have a look.