Effectively the same as this (Drupal 500 Internal server error on login page) but i am unable to bump or comment so resorting to adding my own question.
The site is running Drupal 7.54 php version 7 and uses LoginToboggan module with login.
I am receiving "500 (Service unavailable (with message))" on the user and register pages, this occurs against user/login and ?q=user/login equivalent. Otherwise the site operates normally. There are no PHP errors in the log or browser console so i have nothing to try and trace.
I have amended the .htaccess file to use Rewrite as suggested elsewhere (this was already enabled anyway as the site sits in a subdomain).
Clean URLs are enabled.
I have added error reporting to settings.php to try and pick up on errors i've otherwise missed, nada.
I have used phpMyAdmin to clear the sessions table as suggested on another old thread. This resulted in me being kicked out and unable to get back in.
I can now only access the site db via phpMyAdmin and the site files via ftp (unable to log in as admin as login throws the error)
I tried to run update.php (removing login requirement from settings) out of sheer desperation but this had no effect.
Again in desperation i removed the LoginToboggan module (via ftp, i know) to try and revert the login to core but this had no effect.
I thought the main domain was conflicting in some way so i removed it (it wasn't actually used for anything and had an independent db so i just uninstalled it. No effect.
I really am at a loss, i can't figure out the cause and so have no way to implement a fix..