Problems with defining UPLOADS constant

_{(Converting my comment to an answer.)}

Since you store themes and plugins in that folder as well, try setting the WP_CONTENT_DIR constant instead. (Always prefer absolute paths when possible.)

As to your specific case, why it worked in one file but not in another – this will probably involve more debugging, checking where exactly WP uses which constant, etc. If setting WP_CONTENT_DIR solves your problem, remove UPLOADS and work with it.