Nextcloud 10.0.6: Problem with PHP Update (5.4 mod_php to 5.6 FastCGI)

I’m running NC 10.0.6 on a server with Debian 7.11 (managed with Plesk onyx). I would like to update to NC 11 but I have a problem which seems to be related to PHP.
When I try to update the PHP-version from our actual version (PHP 5.4.45 mod_php) to any other version (5.6.36 or 7.0.30, both FastCGI) I get errors of permission (data-folder and config-folder not accessible).
The owner is set to www-data for all files and folders, which seems to be correct. If I just change the permissions of the above mentioned folders, NC doesn’t allow access claiming wrong permissions.
Does anyone have an idea what I need to change concerning the PHP version?

Probably a missing path in the wrapper script.