4.5 breaks Some Things

Thanks for the reports! I can’t seem to reproduce these issues on any of my sites, but we should be able to get to the bottom of this together:

it breaks the Media Library in thumbnail view, loading icon just spools and nothing happens but the detailed view loads fine.

Could you check for Javascript errors on the page, as explained here:
http://codex.wordpress.org/Using_Your_Browser_to_Diagnose_JavaScript_Errors

when I try to create a Contact Form on a page, the popup window with the options returns an ‘Error 500’.

Could you add the following to your site’s wp-config.php file?

define( 'WP_DEBUG', true );

if ( WP_DEBUG ) {

        @error_reporting( E_ALL );
        @ini_set( 'log_errors', true );
        @ini_set( 'log_errors_max_len', '0' );

        define( 'WP_DEBUG_LOG', true );
        define( 'WP_DEBUG_DISPLAY', false );
        define( 'CONCATENATE_SCRIPTS', false );
        define( 'SAVEQUERIES', true );

}

Your wp-config.php file may already include a line that says “define(‘WP_DEBUG’, false);”. You can remove it, and replace it by the code above.

Once you’ve done so, reproduce the 500 error and then check the wp-content/debug.log file for errors. You can paste the results here. You can then replace define('WP_DEBUG', true); by define('WP_DEBUG', false); in the code above.

Let me know what you find!

Thanks for giving it a try.

Let me know what you find on the other sites!

Do you see any JavaScript errors in your dashboard when trying to load the library?
http://codex.wordpress.org/Using_Your_Browser_to_Diagnose_JavaScript_Errors

If so, could you try to switch to one of the default themes (like Twenty Sixteen) for a few minutes, and check if the errors disappear?