I spoke too soon...
For some reason the above steps did not solve my problem.
In fact, I believe it somehow led to the deletion of my entire /var/www directory
when I tried later to delete mysite3 (/var/www/web3) in ISPConfig.
I think there is a bug in ISPConfig in the way it assigns group numbers.
This is the sequence of events and what happened:
1) I created Mysite1 in ISPConfig (/var/www/web1)
ISPConfig assigned web1 to GID 10001
2) I created Mysite2 in ISPConfig (/var/www/web2)
ISPConfig assigned web2 to GID 10002
3) I installed ossec (a program) and
CentOS assigned it to GID 10003
4) I created Mysite3 in ISPConfig (/var/www/web3)
ISPConfig assigned web3 to GID 10003
This won't work! web3 and ossec cannot both be 10003.
It appears that ISPConfig is not looking at the /etc/group file before
it assigns group numbers. It is simply using an auto increment field in the isp_isp_web table of the IPSConfig MySQL database to generate group numbers. If that is true then it's a BIG problem!
Am I wrong?