Fatal Error when updating seo in frontend

Hello!

We love your extensions and it worked perfectly until recently.

When we save the new Title, keywords, description, etc. it now causes an error which prevents my site from loading or being viewed.

Here is the error we are given

Fatal error: Call to undefined function mb_substr() in /home/invitesc/public_html/plugins/system/easyfrontendseo/easyfrontendseo.php on line 200

Your help on the situation would be amazing:)

All my best
Tiffanie

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