Installation from softacoulous to debain stretch - failure on first start - where is the log?

I’ve installed nextcloud from softacoluos (NC V.15.0.0), the tests where good, the installer did his work well. But the adminsite can’t opened, a failure message with no special hints appears:
Internal Server Error:
The server encountered an internal error and was unable to complete your request.
Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.
More details can be found in the server log.

No meaningfull entry in syslog, nor in apach2-faillog nor in apache2-accesslog for the subdomain. Where can I search some hints, what could be wrong? Database is o.k, could be accessed with adminer
Some more infos about the server:
#Linux version 3.16.0-4-amd64 ( (gcc version 4.8.4 (Debian 4.8.4-1)
#1 SMP Debian 3.16.43-2+deb8u5 (2017-09-19)

PHP Version 7.1.25

MySQL: 5.5.5-10.1.26-MariaDB-0+deb9u1 mit PHP-Erweiterung MySQLi

I would be very grateful for a few hints to the error search

Greetings from the OreMountains