Taxonomy link not working (leads to 404 page)

It seems you do not have a template for your custom taxonomy.
I do not think so single.php will be used for this purpose.
Have a look on this ARTICLE.. Look at the hierarchy,,,

I think in your case you need taxonomy-soortpost.php

Taxonomy hierarchy:

  • taxonomy-{taxonomy}-{term}.php
  • taxonomy-{taxonomy}.php
  • tag-{slug}.php
  • tag-{id}.php
  • category-{slug}.php
  • category-{ID}.php