Issue adding new fields to pages

So, a recent problem I've been having: Every time I try to add a new field to a category, when I click the "Add New Field" button after I've typed in the name, it gives me a "This page isn't working" message, and refreshing the tab doesn't work.

I can open another tab, and when I check, the field I created is there, but… it's a bit tedious to have to open a new tab every time I create a new field.