PP doesn't run after installation

I installed PP successfully, but it doesn't run.

The first message was:
"Error
We are sorry, but ProjectPier failed to connect to the database. Error report has been sent to the administrator so problem should be resolved soon."

I looked in this forum an found:

"To solve the issue, I edited the config/config.php and changed
define('DB_PERSIST', true);
to
define('DB_PERSIST', false); " by marcosdsanchez_thinkclear

Thanks for this, but it doesn't work...

I changed into debug mode and got following error message:

File '/var/www/vhosts/rocoto033.server4you.de/httpdocs/projectpier-0.8.0.3/environment/library/database/adapters/MysqlDBAdapter.class.php' doesn't exists

What goes wrong? What can I do?

Greetings