This block contains unexpected or invalid content

I deactivated the theme and switched to twenty twenty-one.
Also disabled recently updated plugins

The issue still persists

Does the issue persist with all plugins deactivated?

This code was causing the problem. This was in functions.php

Added to Remove Query Strings from Static Resources as per this article. [https://www.keycdn.com/support/remove-query-strings-from-static-resources ]

function _remove_script_version( $src ){ 
   $parts = explode( '?', $src ); 	
   return $parts[0]; 
} 
add_filter( 'script_loader_src', '_remove_script_version', 15, 1 ); 
add_filter( 'style_loader_src', '_remove_script_version', 15, 1 );

It was added ages ago. Not sure why it broke now.