Can't Edit Post Categories

My default category is called “miscellaneous.” I rarely use it, since it’s only for posts that don’t fit anywhere else. Lately, when I create a new post, all the categories start off unchecked, including “miscellaneous.” I go in and check all the categories I want. But when I save or publish the post, it’s categorized under everything I checked off and “miscellaneous.” If I go back in to try to fix this in Edit or Quick Edit, all of the categories are unchecked again. So I have no way to remove any unwanted categories from a post.

I’ve tried different browsers, but the checkmarks still don’t show up. I’ve tried deleting the category and creating a new “miscellaneous” category, but any new posts I make will still be in “miscellaneous” or whatever the default category is called. I could change the name of the category to “articles” or something else generic, but I really don’t want a category that just has evert article I post in it.

My website is inkandpixelclub.com.

I would really appreciate any suggestions, as this is getting very frustrating.