Notice: ob_end_flush(): failed to send buffer of zlib output compression (1) in

It solved when switched off zlib.output_compression in php.ini

zlib.output_compression = Off

WordPress attempts to flush the output buffers on shutdown. It fails because you have already called ob_end_flush().

You should be able to keep compression on, and simply unhook the flush action:

remove_action( 'shutdown', 'wp_ob_end_flush_all', 1 );

You can now call ob_end_flush() manually, and keep zlib compression on.

I found a particular plug-in was the cause on one of our client's WP sites.

In this case it was caused by the "NextGEN Gallery" plug-in, but weirdly simply deactivating and then activating the plug-in resolved the issue.

For anyone else having this problem it would be worth looking for suspect front end facing plug-ins and trying the same. If you find that the issue comes back when the culprit plug-in is reactivated you should file an issue with the plug-in author.

I wouldn't recommend disabling the wp_ob_end_flush_all() function entirely, and I definitely wouldn't turn off zlib.output_compression in your php.ini file. Here's a better approach that replaces the source code causing the issue, and preserves the underlying functionality:

/**
 * Proper ob_end_flush() for all levels
 *
 * This replaces the WordPress `wp_ob_end_flush_all()` function
 * with a replacement that doesn't cause PHP notices.
 */
remove_action( 'shutdown', 'wp_ob_end_flush_all', 1 );
add_action( 'shutdown', function() {
   while ( @ob_end_flush() );
} );

More details on the cause, and why this is probably the best approach can be found here: Quick Fix for WordPress ob_end_flush() Error