Password reset causing 404
Something is intercepting the submission to the user password reset form. 

The form should post to the user.reset.login route (/user/reset/{uid}/{timestamp}/{hash}/login), handled by UserController::resetPass(), then redirect to /user/{uid}/edit?pass-reset-token={TOKEN}. 

This could be a number of different things which you'll have to troubleshoot (e.g. Rules performing some redirect logic on login, an altered form submit handler, Apache redirect, a module performing some redirect logic on a subscribed event, etc.). 

Given that the destination URL is essentially appending "/admin" to password reset form URL that should probably clue you in on what is causing the problem.