I had a 404 problem for pages, but not the is_tax() views. Turns out the plugin wasn’t supplying a value for the rewrite:
‘rewrite’ => array(‘slug’ => ”)
..like it should be like this: ‘rewrite’ => array(‘slug’ => ‘designtype’).
Another mildly-related problem was the first argument to both ‘register_post_type’ & ‘register_taxonomy’ must be lowercase.
Hope that’s reasonably helpful.. likely not an exact solution