Link to a page where the issue can be seen: hidden link when clicking on an activity you will get an 404: hidden link
I expected to see: a page with custom content filled
Instead, I got a: 404
I tried fixing it with your advice in this post https://toolset.com/forums/topic/custom-post-types-and-related-content-missing/ but it didn't work. Also i tried making an new post type with the same slug (which i think it is the right one but don't know for sure). i've updated the toolset plugins and (de)activated all others.
The slug can actually be found if you check the database.
If you're able to recall the title of any of the posts under this post type then you can check the wp_posts table in the database under the post_type column.
There you should be able to see the slug. If you would like for me to check please let me know. However it would require the installation of the WP phpMyAdmin plugin.