WP 5.5 getting PHP Notice

This may be a plugin or theme conflict. Please attempt to disable all plugins, and switch to the default Twenty Twenty theme. If the problem goes away, enable them one by one to identify the source of the problem.

If you can install plugins, install Health Check. On the troubleshooting tab, you can click the button to disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site.

Many thanks, will check this.

I enabled troubleshoot mode and still getting the same errors with no plugin enabled and with default theme…

Ok, try downloading WordPress again, access your server via SFTP or FTP, or a file manager in your hosting account’s control panel (consult your hosting provider’s documentation for specifics on these), and delete then replace your copies of everything on the server except the wp-config.php file and the /wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings.

Some uploaders tend to be unreliable when overwriting files, so don’t forget to delete the original files before replacing them.

Thanks, will try this as you described.
I forgot to mention that I already re-installed version 5.5 from the dashboard but that didn’t solve the problem and that the health check doesn’t find anything wrong and is labelled “good”.

Did that but, unfortunately, getting same results.

I checked that line and refers to that:

/**
 * Flush all output buffers for PHP 5.2.
 *
 * Make sure all output buffers are flushed before our singletons are destroyed.
 *
 * @since 2.2.0
 */
function wp_ob_end_flush_all() {
	$levels = ob_get_level();
	for ( $i = 0; $i < $levels; $i++ ) {
		ob_end_flush();
	}

Maybe something to do at server level?

Possibly, is zlib installed on the server? You may need to contact your hosting provider’s support.

Yes, that was it!
Host corrected the issue and it works fine now.

Many thanks for your help!

@dsl225 what did your host do to fix it? I am having something similar

@dannyogolo
Don’t really know what they did exactly, but I sent them the error message plus related code and asked them to check whether zlib was installed, as @macmanx suggested above.

It took them less than half hour to solve this but didn’t provide any further details. I presume this library was simply not active/installed on server.

UPDATE
My host sorted mine out. Had to deactivate Ninja form and its add-ons and an old Paypal plugin. Everything worked, but I have to downgrade WP from 5,5 so I can continue using ninja