Is it possible an interface .php file was damaged during transfer/decompress? (just grasping at straws here)
I guess you would see some kind of php error then. But you can redo the update to ensure that all files are correct.
tar xvfz ISPConfig-3-stable.tar.gz
php -q update.php
If this wont help, you might consider to contact us for remote login support so we can check that on your server. Our support can be reached here: