Hello,

You have this error message because you don't have mbstring activated on your server (it rarely happens...)

So, in 3.4.0, i will check if still some mbstring function with no control to prevent this error message.

You can ask your hoster to activate mbstring or you can remove all mb_ prefix found in the code (for example, replace all mb_substr with substr).
But substr is working as expected only with latin language.

Regards,
Cyril