Yes, there were errors regarding that the executable of "suphp" could not be found. I could fix it by installing suphp from the official Debian package. After I got suphp to work that way, I encountered new problems, however. One of them was that ISPConfig did not write changes to its Vhost-file anymore. And even though php pages were delivered via suphp, suphp-statements in the Vhost-file were reported as errors by the apache. For the moment, I undid all the changes I made and uninstalled suphp.