Today we updated the install.php and update.php script and added a warning that ISPConfig 3.0.5.4p9 is not compatible with PHP 7 as system PHP (no problems with PHP 7 as additional PHP though). This is not an update, so no need to update existing installations.
If you use PHP 7 as system PHP (e.g. when you use the brand new Ubuntu 16.04), then please use the ISPConfig 3.1 pre release:
https://git.ispconfig.org/ispconfig/ispconfig3/repository/archive.tar.gz?ref=stable-3.1
