Dashboard not accessible since upgrading to 4.2.2

Having the same problem (500 internal server error) when trying to access dashboard, and even my site is down. Haven’t tried to install a previous version yet (it’s just too late, I’ll give it a try tomorrow).

I got in touch with my hosting and the answer was: “In order to make the Plugin work, you should set permissions to 755”.
I hope it can help you. Cheers!

Does this happen when updating manually? To do so, log in to your site via FTP or CPanel, delete the existing Jetpack plugin folder, and reinstall the plugin manually as explained here:
https://codex.www.ads-software.com/Managing_Plugins#Manual_Plugin_Installation

You’ll find the Jetpack plugin folder here:

wp-content
   plugins
     jetpack - DELETE THIS FOLDER

If you’re not sure how to do this, you can ask your hosting company for help. They should also have documentation explaining how to edit or remove files from your site using FTP or a File Manager in your admin panel.

If you still get a 500 error, 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 );

}

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

Thanks!