Hi Lauren,

It looks like your editor left some blank space (space, tab, new line) either before the first <?php or after the closing ?>

Use a plain text editor like Notepad or (better) Notepad++ or Crimson editor. There are hundreds of them around. Do not use an HTML editor when you edit php files.

If you have a backup from the _main.php file, you'd better start again. If you don´t you'll have to open the file, search and delete the whitespace.

Good luck

omg! I had already read this answer in others topic, however I did not find the whitespace. Now I have used winmerge, I have modified the text, saved and uploaded: it is all ok!
(I ask myself why until yesterday I used block notes and I didn't have problems.... mystery)
Thank you very much and happy new year! ;)