Registration link in Tikicorp
The problem is happening because the log-in form wasn't fully updated when Tiki upgraded from Bootstrap 3 to 4 (the HTML and CSS of dropdowns containing links was simplified), so that needs to be corrected in Tiki's code.
In the meantime, you can add this to the CSS Customization textarea on tiki-admin.php?page=look:
.dropdown-menu li.dropdown-item::before { z-index: -1; }
I believe that should enable the link to work.
-- Gary