For first time i have change the setting about the "Number of recent tracks:" in widgets -> settings. For first time i see fatal error and now everytime when Ι update something from settings I see the same error.  But now everything in panel is not working correctly. The start/stop button from autodj is also not working.

Ther error is that:
Fatal error: Call to a member function get_name() on a non-object in /usr/local/centovacast/system/class_XML.php on line 632

Try the latest build.  I really doubt it has anything to do with changing the number of recent tracks (as the error you quoted is an XML configuration file error, whereas the number of recent tracks is stored in the database) but the latest version has some XML parsing fixes, plus it generates more detailed error messages when an XML configuration file has errors.

If you still get errors after upgrading, click the "Details" link and post the details shown there and we should be able to identify the problem.