I have the same problem, and I'm an experienced IT developer.
I inserted a little code in /www/sportboerse.ch/de/alle/kleinanzeigen/oc-includes/osclass/classes/database/DBConnectionClass.php in the statement on Line 271. It now contains this:
$message = 'Osclass database server ' . $this->dbHost . ' is not available. <a href="
http://forums.osclass.org/">Need more help?</a></p>';
I've inserted ' . $this->dbHost . ' in this statement. It now shows the DB-Host Name which is unavailable.
From my hoster I got two host-names. An internal one which is sportboe.mysql.db.internal, and an external one whcih is sportboe.mysql.db.hostpoint.ch.
Both do NOT work.
I was able to install osclass without any problems on a local server.
Next I've checked function _connectToDb within the file mentioned above already. I've seen, that you are using mysqli to connect to the DB. I wonder why this call contains only three parameters instead of the four the mysqli manual shows.
Have not invested more time so far to find out what's really wrong.
So it would be really helpful, if anyone could tell us about the required correct form of the HOST name entry in the installation form.
Thanx for any help, it is highly appreciated :-)).