Originally Posted by xyhfna
This is going to sound really stupid, but... I tried an older version of ISPconfig... and it worked first time. I think 2.2.9 may be broken for older servers (i know it evidently shouldnt be!).
If older ISPConfig versions install fine, but the newest one isn't, I guess the package g++ is missing. This is required by the PHP version in ISPConfig 2.2.9.
apt-get install g++