Hi, my configuration first:
I have a backup of my Piwigo galery downloaded from my Online Server (now terminated).
The Server Configuration was as follows:
  PIWIGO 2.9.4
  Operating System : Linux
  PHP: 7.0.33 [2018-12-10]
  MySQL: 5.5.5-10.2.19.MariaDB [2018-12-10]
  Grafic Library: GD 2.2.5
The Galery had 502 albums and 8752 pictures
The backup file size was 4.6 GB and was exported on WIN7 folder by the file "20181216_piwigo.sql.bz2".

Now my new target system is a Raspberry PI3 B+ with 32GB ROM and 120GB SSD.
I installed by now
  ServerType : MariaDB 10.1.37
  Apache/2.4.25 (raspbian)
  PHP: 7.0.33+deb9u3
  PHPMyAdmin. 4.6.6deb4

After a successful start of Apache2 and phpMyAdmin and the import into
a new database (by "20181216_piwigo.sql.bz2") I can see all my files of the old galery (pictures, albums, rights) with the tool phpMyAdmin.

Now my problem: I have no access by my standard browser to the galery. Something is obviously not configured or installed.
E.g.: I cannot find the file "DocumentRoot".

Please help.

regards
  Alhard