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

Hello,

yes, most likely, you (or someone else) have modified the file in question (english.php) and there is some extra character on line 1 before <?php. This may break things.
Check that there are no extra characters before <?php (even empty ones that may not be visivle!) as resave the file.

Also, please note this post does not belong to this forum or check the forum rules.

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,

yes, it is possible this error appears when JoomSEF is activated, because it loads the translation file to be able to use it for URL localizations. (earlier then it would be loaded normally, when headers were already sent, so the error does not show up)

Also, it is possible, the extra character is not visible in the text editor you are using. Try to view the file in other editor - the best would be some basic commandline editor or notepad. (the charater maybe in certain encoding and more advanced text editors may render it invisible) You may also delete the whole part <?php and then write it again resave -- make sure the encoding will be Latin1 (or ASCII).

However, based on the message "output started at /home/..path../components/com_mtree/language/english.php:1" I am pretty sure this is the cause of the error - I have already seen such earlier.

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