Image uploader sending me to 404

I’m having trouble with my image uploader. The image uploads to my database, but when I hit ‘insert into post’ I get a 404 in the ‘add an image’ window. I also tried uploading the image, then grabbing the URL and adding it into the HTML. This resulted in showing the image in the blogpost preview, but when I hit ‘publish’ I get a 404 error in the window.

I can publish text no problem, and I can also publish a ‘featured image’ fine. It’s just a problem with the images in the posts. I have checked in settings and ‘store images in this folder’ is set to wp-content/uploads – and have checked all of my images are in this folder where they are supposed to be.