UPLOADS constant not working in WordPress Multisite

I think that you’re saying that you have moved your “uploads” directory up a level such that it now exists within your installation root, alongside /wp-includes, wp-config.php, etc.?

If so, you likely need to update the rewrite rules in your webserver configuration. By default, a multisite .htaccess configuration has a rule like

RewriteRule  ^[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*) $1 [L]

This says “if the URL path begins with /somesite/wp-content/whatever then serve the file located at {ABSPATH}/wp-content/whatever” (and the same for wp-admin and wp-includes).

Because uploads is not factored into this or some other rule, no attempt is made to resolve the path to a file, and instead WordPress will end up processing the request as though that path is a permalink.

Add another rule above or below that line in order to account for your new location:

RewriteRule  ^[_0-9a-zA-Z-]+/(uploads.*) $1 [L]