Categories Show the Wrong Posts

I’m having troubles with category archives showing posts that are not in that category. I have searched google and the forums and found tons of people with the same problem as this, but the solutions there are all the same: disable your plugins, try the default theme. I have disabled plugins and tried the default theme, and yet I still have the same issue. I can’t find any solution.

Basically, I have a number of categories and subcategories on my site. When I display the archive page for a category with no children (example: mysite.com/category/a/b/c/) it displays the posts in category c correctly.

When I try mysite.com/category/a/b/ it does not display the posts in category b correctly. Instead, it displays a whole bunch of posts totally unrelated to that category, followed by the posts from category b.

This happens regardless of the theme or the plugins I have. It happens if I search posts in the backend too, not just the front end. Is there anything I can do to fix this besides making individual category pages for each archive (because there are a lot)??

Edit: Something else I just noticed now that may(?) be related to this problem: certain categories are giving me a server 500 error when I try to load them. This happens if I directly link to them on the front end of the website, and it also happens if I click on the link to view the posts in the category from the backend. Even though the category is not empty and it shows me a postcount, I can’t even load the backend page that shows me what posts are in the category. What could be causing this???