Can you reach out to the hosting provider and ask to have a look at this, please? This is all about some settings on the server side and there's nothing we can do on the plugin to make this function appear on the server.
However, the hosting is able to check things out and see why is this mb_ereg_replace_callback function still missing, even if the Multibyte String Extension is enabled.
Please give this a try and tell me how it goes.