FYI: In preparation for multiserver setup, I've updated server #1 from debian squeeze to debian wheezy.
Upgrade went fine, however after I rebooted I couldn't access ISPConfig control panel, I was getting Internal Server Error. Also, websites were showing 500 error. After some troubleshooting I had to do the following to be able to access ISPConfig control panel
Commented out the following line from both php.ini files:
If anyone has same issue with upgrading to debian wheezy, this post could perhaps help.