Custom contraint validator: show an error message with a link
You shouldn't chain up placeholders / use HTML inside placeholders. And for URLs use the :placeholder notation with double dots
 $notUniqueURLBasis = 'The basis URL needs to be unqiue but the set basis URL %basisurl is already in use: &lt;a href=&quot;:existing_url&quot;&gt;@link_text&lt;/a&gt;';

See https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Component%21Render%21FormattableMarkup.php/class/FormattableMarkup/8.2.x
and
https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Component%21Render%21FormattableMarkup.php/function/FormattableMarkup%3A%3AplaceholderFormat/8.2.x