How to fix “Call to undefined function mb_strpos()” PHP error in BogdanFix plugins?

If you are experiencing this error message after installation of Handsome Checkout or One Click Upsells, it means the PHP build on your hosting server does not have the important “mbstring” module included.

Usually there is a quick fix for this: to fix that you need to contact your hosting support and ask them to enable “mbstring” extension for your sites.