I upgraded my server to debian 9, mariadb and php7.0, mainly because my laravel project requires it now. After switching the configuration of apache via ‘a2dismod php5’ and ‘a2enmod php7.0’, I only see a blank page. The console tells me that the server responds with 503. Sadly nothing shows up in the logs of apache or nextcloud itself (even with debug level 0). If I rename config.php and make it therefor inaccessible, the post-install screen shows up. My other pages and projects work just fine so the configuration “should” work for nextcloud. I have not changed any ownership or groups of my nextcloud folder.
It should be safe to remove (not purge) all php5* packages, if you want to go back you can install it again. (Note though, that you will need an extra repository like this one because Debian 9 stretch only includes php7.)