You were right!
Thank you very much!
The encoding was utf8, and was not showing the gibberish text..
I can't thank you enough!

Hello, thank you for your answer, unfortunately, I checked that, but there isn't anything in front of the <?php tag. This warning appers when I have joomsef installed and activated. Sorry about posting in another category, I though this warning appers because joomsef, and I searched for help.

Hello, could you please tell me what is this warning from?

Warning: Cannot modify header information - headers already sent by (output started at /home/..path../components/com_mtree/language/english.php:1) in /home/..path../components/com_mtree/mtree.php on line 1473

thanks in advance