Pluggable Error

Hi, I have a client who is receiving this error message at login.
Warning: Cannot modify header information – headers already sent by (output started at /home/content/77/9514377/html/wp-blog-header.php:2) in /home/content/77/9514377/html/wp-includes/pluggable.php on line 876

I disabled all the plugins and that did not work. I replaced the pluggable.php page from a fresh copy of wordpress. Any ideas what to try next. Do I need to disable each plugin one by one? There were other posts talking about white space? The theme they are using is a custom one. https://www,boxleafdesign.com. Thanks.