Thank you, Till. What you suggested worked just fine.
To those who may have a similar problem in the future please try this sequence of commands to repair your installation:
tar xfz ISPConfig-3-stable.tar.gz
php -q update.php
Then just follow the defaults in the update script if you are using a single server to run all the services that this package administers. Note though that if you have made changes to postfix for smtp relay, you will have to redo those changes manually.