I'm using Simple Custom Post Type UI. I was considering writing my own custom implementation, but after looking into the plugin's code I realized it's just a UI wrapper (just like you mentioned it), so there is really no reason re-creating the wheel.
Now, I'm trying to use your plugin with the mentioned above and I can't seem to figure out why it doesn't work. The conditions in the is_scpta_post_type() are satisfied, is_custom_post_type_archive() returns true as well... but my page gets broken after the header and navigation loads, right in the post content area. I also checked the HTML source for the body classes and I don't see any custom classes to be added by the scpta_body_classes()...
I tried adding the additional templates, but that outputs broken pages the same way as the index.php.
The fact that my custom post archive page doesn't return 404, already tells me that something works, but I don't know where exactly it breaks. Any ideas on what I should look into?
P.S. I followed you FAQ and re-saved permalinks just to be sure.