this seems like a complicated problem, and i think i am just understanding it.
i'm only guessing here, but i think having a WordPress installed in a subdirectory named '2015' makes the main WordPress install in the main directory confused, and it might have an easier time understanding a Word instead of a number. and since the main WordPress has a .htaccess file it takes precedence over any subdirectories. when someone visits example.com/2015/ the main WordPress thinks you are trying to visit a blog post or page on the main WordPress install (but can't find it) and then redirects you to the home page.
Maybe having a distinct name (a word not a number) for the location of your secondary WordPress install will help solve this issue.
since /2015/wp-admin/ works, try this:
Go to your Admin dashboard for the 2015 install, and go to Settings > General, under 'Site Address (URL)' type in http://example.com/seconday/ - make sure to come up with a Word for a name and use it where i have used the word 'Seconday', and double check that it is not currently in use on the main website. (don't change WordPress Address (URL) at all)
if this fails, /2015/wp-admin/ should still work fine, so you can change it back.