Images upload to correct folder but return 404's

When I try uploading images, it uploads them correctly to the FTP site (in the wp-content/uploads/[year]/[month] folder). Via FTP, I can download and view all of the crop sizes and they all look normal. However, in wp-admin they show up as broken images, and when I go to view the image in its own tab, I get a 404 page. Web inspector in wp-admin is showing 404s for the thumbnails too, even though the URLs look correct.

Any ideas? In my wp-config, I have defined the following, in order to skip through the “enter your connection details” screen when adding plugins:

FTP_BASE
FTP_USER
FTP_PASS
FTP_HOST

However, I’ve done that on other sites on this server and it hasn’t affected image uploads.

My permalinks are set to

/%post_id%/%postname%/

Halp??