so find this in your /etc/php.ini
; open_basedir, if set, limits all file operations to the defined directory
; and below. This directive makes most sense if used in a per-directory
; or per-virtualhost web server configuration file. This directive is
; *NOT* affected by whether Safe Mode is turned On or Off.
if it commented out un comment it and add
If you were going to put that command in PHP.ini i would probably specify what [website_path] and [website_domain] are because only the ISPConfig web interface knows the true path.
see if that fixes it, if not then we would have to try something else.