Cannot add images to Media Library, posts or pages after installing 4.0

I have updated to 4.0 and since doing that, I cannot add images to the Media Library, posts or pages.

I get no error message and the image appears to upload successfully. However, instead of seeing the preview/image in the Media Library, post or page, I get the missing image symbol. I can, however, see the image if I go to edit it (but editing it doesn’t then mean the image appears).

I haven’t changed anything else. I have tried disabling all plugins before trying again but it still doesn’t work. Help?
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