This where the errors wich occurd within the log file of the typo3 sites:
[Tue Feb 12 14:14:28 2008] [error] [client 123.45.678.90] SoftException in Application.cpp:214: File "/var/www/typo3_src-4.1.5/index.php" is not in document root of Vhost "/var/www/web123/web"
[Tue Feb 12 14:14:28 2008] [error] [client 123.45.678.90] Premature end of script headers: index.php
For this reason i disabled/enabled php for the typo3 site to force ispconfig to write a new vhost file, which solved the problem... i only don't know how this could happen. But ... I am happy that the typo3 sites are up and running again in no time