Fatal Error

[AquilaTempestas]

I've had this exact problem before and it was supposedly due to a plugin, but I don't know which one. 

The last time it happened the problem fixed itself up within 24 hours. Any advice?

Fatal error: Call to undefined function IPS\mb_internal_encoding() in /home/pokemani/public_html/theartistszone.com/init.php on line 125

[AquilaTempestas]

So I ended up getting a full back up done and the problem now changes to

[ Fatal error: Call to undefined function IPS\mb_substr() in /home/pokemani/public_html/theartistszone.com/init.php on line 98 ]

I haven't made any changes at all over the past month so I don't understand what has caused this error to arise. The site was working perfectly fine until the afternoon of the 17th of September.