Problems with current mysql- and php versions

[Playaowl]

Using a DigitalOcean Ubuntu 14.04.5 x64
Used the install_dependencies.sh setup (but also tried a LAMP system according to the local installation readme).
The engelsystem shows up, but the registration page either from the start page or from the admin-add user button is a 0 byte page, totally empty.
Besides a bunch of errors of redefined constants because config-sample.default.php is included before config.php
the apache2/error.log shows:
[Sat Jan 21 23:16:17.560400 2017] [:error] [pid 8791] [client x.x.x.x:52724] PHP Fatal error: Call to undefined function select_angeltype() in /var/www/html/engelsystem/includes/controller/guest_login_controller.php on line 47, referer: http://x.x.x.x/?p=login
but on first sight all the files appear to be there.

Any idea whats wrong? Any chance to get it working against current mysql and php versions?
Regards
Owl

Update:
added select_angeltype() as a copy of select_angeltypes() to Message_model.php.
After that I get a sql error, pointing out that the construction of the statement is only allowed without sql-mode ONLY_FULL_GROUP_BY
It seems that the current versions of mysql (5.7) uses this sql-mode as a default (changed since 5.5).