Blank page after install

[csabix]

Hello all

As the title says, I get a blank page after I install Roundcube.
Tried it with the one that comes with Debian and with a freshly downloaded edition, too (1.3.5).
My server is running Debian Stretch (9) with MariaDB, php7, apache 2.4.
I used this tutorial: https://www.howtoforge.com/tutorial/roundcube-installation-on-debian-8-jessie/
I suppose there are some fundamental differences between Jessie and Stretch.

Can anyone help me out in troubleshooting the issue?

Thank you!

[SKaero]

Whats in the Roundcube error log?