I realized that I wasn't able to log in in any way. But this was seeming a problem of some older versions of Joomla. They allow you to enter any value you want as timeout but if you enter a too high value you will be not able to log in again. Fortunatelly I could access all the source files of the CMS via FTP and I also had the access-data like username and password and so I could change the timout value to another one in two source files in which it is neccessary: the index2.php and the index3.php file. You can find that solution in the troubleshooting-section in the Online-Book "Joomla!" from http://www.galileocomputing.de.