I encountered a strange bug:
After I changed my database password to "ps*nq$TsUbxaJyit" I got this error during setup:
Access denied for user 'xxx'@'xxx' (using password: YES)

In the setup.log I found the following message:
NOTICE: Undefined variable: TsUbxaJyit in /var/www/xxx/html/fotos/zp-data/zenphoto.cfg.php in Zeile 30
require called from index.php [755]

Apparently the $ caused Zenphoto to interpret a part of the password as a variable. So just a part of the password was used to login to MySQL.

That's probably because the config file is actually "sort of parsed" and not just included as plain php file so it probably just mistakes that as a variable. See the root index.php.