Recently we started to have 404 trying to access our "single downloads" pages. This problem arises only on some of the installations, and very spontaneously.
It seems like the issue for this FAQ entry:
Getting a 404 error?
To get rid of the 404 error when viewing a download, you need to resave your permalink structure. Go to Settings > Permalinks and click "Save Changes".
But this advice isn't working.
Direct URLs, like website.com/?p=XXXX, are giving 404 as well.
What can be cause of it?