Attempting to add post/image breaks the website

I’m having a very strange problem.

Whenever I try to add a post (or more specifically, add an image to a post), the dashboard freezes and then I’m not able to do anything in wordpress. I can’t even log out. Usually it ends up with a “connection timed out” error from firefox.

So I just exit out of the tab. But then I can’t access the website, I can’t access the wordpress dashboard log in screen, I can’t even access my server’s control panel. In fact, I can’t access the website from any computer on my home network. Even using different browsers results in the same problem.

It gets weirder. While this is happening on my computer, other people can view the website. In fact, my business partner can log into wordpress. But if he tries to update a post, he runs into the problems described above.

The only real way I can get back into the wordpress dashboard is to clear my cookies, cache, internet history, and restart my browser. But if I attempt to update a post, the whole set of errors starts over again.