[TYPO3] Problem with recode functions

[Andreas Balzer]

Hi,
I get the following error on my website: Fatal error: Call to undefined 
function recode_string() in 
...../typo3_src-4.2.0beta3/t3lib/class.t3lib_stdgraphic.php on line 2013

However I have set T3 to use mbstring in the install tool.
What can I do?

Greetings,
Andreas