Please double-check that you really disabled suExec in ISPConfig.
What's the output of
ls -la /etc/apache2/vhosts
? Are there Vhosts_ispconfig.conf files with a date appaneded to them? If so, take the last one, rename it to Vhosts_ispconfig.conf (please back up the current Vhosts_ispconfig.conf) and run
What's the output?