I'm having this same issue with every one of my wordpress sites. It started happening two days ago. So far I've upgraded all my WP sites to the current version. I removed almost all the plugins. Right now I'm removing old themes.
I primarily use OptimizedPress and StudioPress themes, but had a bunch old ones just hanging in there.
My errors look like this:
Warning: Cannot modify header information - headers already sent by (output started at /usr/lib/php/ini:49) in /home/authorst/public_html/wp-includes/pluggable.php on line 866
Some of my WP sites look fine on the fronted and on the backend the admin is either not reading the style css or it's throwing this type of blank space error. The ones throwing the error... I can't do anything with. There's no content displayed.
Some of my WP sites are throwing the error on the fronted too. It's weird.
Any ideas or thoughts... and especially a fix are welcomed!