I was having a similar problem with my site. I searched high and low and followed all the recommendations (disabled plugins, checked wp-config.php, checked for trailing blank line in wp-login/wp-admin php files, etc). It wasn’t until I went into my DB and reverted to the default theme that I started taking a hard look at the theme I was running (boldlines). I noticed there was a blank line at the end of the theme’s functions.php. I removed the blank line and everything started to work just fine. Perhaps this will help someone else.
Cheers,
Al
