Please refer to https://tracker.phpbb.com/browse/PHPBB-17700 for the environment description.

There are two databases now that can be used alternate with 2 different config.php

The fresh installed database with the installer is working for the admin so far.
As admin it is possible to create a new thread in Your first forum and to post there. The posts can be read.

But after registering as a new user the themes are shown for the new user, but when clicking on a theme this Error is shown:

SQL ERROR [ mysqli ]

Unknown column 'bbcode_group' in 'SELECT' [1054]

This is basic functionality and the error is surprising.