[Typo3] Fatal error - missing function ctype_digit() - typo 3.8

[Golo Königshoff]

Hi!

I am working on a site with typo version 3.8

Whey I try to insert a content element for a mail form, i get an error 
on the frontend:

Fatal error: Call to undefined function: ctype_digit() in 
/srv/www/typo3_src-3.8.0/typo3/sysext/cms/tslib/class.tslib_content.php 
on line 1577

Trying to insert a mail form on a fresh installation works.

Does anyone have an idea where i can start searching for the reason of 
this error? Time is running out - 20 hours left to initial site-launch.

:-/

If you need additional information like installed extensions etc., i 
will provide immediately.

THANKS a lot
Golo Königshoff