TYPO3 Core

Did you encounter the same error message in log?

Where can I find the log?

In your PHP error log file. If you don't know, where it is located, ask your admin or hoster.

You could also enable display_errors in php.ini, to see the error message (which is really important in such blank-page errors).
See also http://dmitry-dulepov.com/article/blank-empty-page-in-typo3.html

Here is the error message:

Fatal error: Call to undefined function filter_var() in /var/www/vhosts/hsbk-oberhausen.de/httpdocs/typo3_src-4.2.11/t3lib/class.t3lib_div.php on line 1555

Are you <b>really, really, really</b> sure that you are running at least PHP 5.2.0? I can't really believe this.
http://www.php.net/manual/en/function.filter-var.php says it exists since 5.2.0.
Is your PHP self-compiled? Maybe there's a switch to exclude the filtering stuff?

You could also install the PECL filter library, which seems to do the same for PHP 5.1

Yes I'm sure.

Suse 9.3 came with PHP 5.0.3 and I had to upgrade. But I don't recall whether I downloaded a rpm or did it from source.

I'm quite sure, you use a somehow weird version of PHP 5.2.0

Please try to use the PECL filter library - or may be you somewhere find a PHP implementation of filter_var()