I have one site with a very strange issue that I have been fighting for months and I'm clueless. All I know is that with CiviCRM disabled, the issue does not occur.
When I attempt to log in and submit the login form, instead of being logged in I get redirected to some random page and 404. For example, it will say
Page /robots.txt not found
And I'm not logged in. The only way to log in is to disable CiviCRM via drush. Then everything works perfectly.
It's something in the DB because I can restore this DB to a fresh
civicrm.settings.php and the error persists.
The DB did go through an upgrade from D6 w/Civi4.5 to D7 w/Civi4.6, which is when the strange behavior started.
Thanks for your time and insights.