Well this error is no fun:
Fatal error: Unknown: Cannot use output buffering in output buffering display handlers in Unknown on line 0
The error is accompanied by the dreaded WSOD (White Screen of Death).
But it may not be as scary as you think.
I started getting this message on my newest WordPress site when trying to view posts with a lot of comments on them. I only get the error if trying to view the page while logged-in to the site as the WordPress admin account. If I log out and view as an anonymous viewer, all is fine.
I’ve narrowed down the issue to two things:
1) using the WP Super Cache plugin
2) the post has over 180 comments
There are a few ways to cope, but I don’t have an actual fix:
1) Disable (temporarily) WP Super Cache – everything works fine with the cache disabled
2) While Super Cache is disabled, or by going through the Dashboard Comments page, delete some old comments from that page so that there are 179 or fewer comments
So no need to panic as these work arounds will get you through the day. But if you know a real/permanent fix, please leave a comment and fill me in on the secret.