Undefined index: mbstring…

Hello,

It seems debug mode is enabled in your WordPress.

Please, disable it by updating the file wp-config.php:

define('WP_DEBUG', false);

If the problem persist, please let me know.

That is correct. The message disappears when debugging is disabled.
It is hard to understand, though, why debugging result in this, and only this, message.

Hello,

Could you please forward your debug.log file from the wp-content folder to [email protected]? For a better follow-up, please drop a line here after sending an email.

debug.log sent as requested