Hello WuChEn
The problem is just that you packed the files inside the folder
so your tar.gz file is packed as a Joomla_2.5.1_de folder and not as the files them self.
if you just mark all the files in the Joomla_2.5.1_de folder and pack those into the tar.gz file, then the installation will work
You can find your package here http://packages.kazi-networks.com/sw/Joomla_2.5.1_de.tar.gz packed correctly.
how ever its installed with "WuChEn" as user, so it doesnt take the info thats typed into imscp under the installation.
in the sql.sql file on line 1657 it needs to be:
[code=php]INSERT INTO `users` (`id`, `name`, `username`, `email`, `password`, `usertype`, `block`, `sendEmail`, `registerDate`, `lastvisitDate`, `activation`, `params`) VALUES
(42, 'Super User', 'admin', 'noreply@i-mscp.net', '8d0a66712dc8a5ae98de5fe8d63edaf3:Sa5g32TJh63mJevap3plKb3W3kFE9cR1', 'deprecated', 0, 1, '2012-02-13 17:14:33', '0000-00-00 00:00:00', '0', '');[/php]
since its in the joomlade file, asks it to rename the user "admin" into the info you type in under the installation.
I have added it to the file i linked to as well.