‘headers already sent’ (already checked FAQ)

I’ve had a problem editing my theme directly from v2.7 back end. Error message is

Warning: fopen(C:\Sites\Single39\ammarcarl\webroot\news/wp-content/themes/black-lucas/style.css) [function.fopen]: failed to open stream: Permission denied in C:\Sites\Single39\ammarcarl\webroot\news\wp-admin\theme-editor.php on line 55

Warning: Cannot modify header information – headers already sent by (output started at C:\Sites\Single39\ammarcarl\webroot\news\wp-admin\theme-editor.php:55) in C:\Sites\Single39\ammarcarl\webroot\news\wp-admin\theme-editor.php on line 70

here’s line 55:

$f = fopen($real_file, ‘w+’);

esmi kindly pointed me to the correct WordPress FAQ troubleshooting section (here). However, I’ve tried the common solutions to the headers already sent message (deleting spaces, re-installing fresh file, even re-installing whole theme) and the problem persists.

I’m able to edit the files with a text editor, but error persists if attempting directly from wordpress. No clue on what the problem could be.