Page and Post return 404 with custom taxonomy

OK, so you’ve used / as you taxonomy slug. It means that URL for your term is:

http://example.com/my-sample-taxonomy-term/

And for pages it will be:

http://example.com/my-sample-page/

As you can see, there is no way to guess, what type of post should WP search for.

WordPress processes the URLs looping through registered Rewrite Rules and matching given URL against regular expression assigned to current rule.

That means that in your case WP will take first rule that will match correctly and try to display that type of object. And because there are two different objects registering rules that are in conflict, then you’ll get 404 errors for some of them.