Hard to die PHP Warning

Dear guys at support.
I’m getting this warning (after enabling the debug command) when trying to access my wp-admin page:
Warning: Cannot modify header information – headers already sent by (output started at /home3/xxxxxx/public_html/wp-config.php:1) in /home3/xxxxxx/public_html/wp-includes/pluggable.php on line 1195

where xxxxxx is my user

I’ve spent two days doing research and trying different solutions.
1. I checked for white spaces on both functions.php (child and father themes)
2. Also checked that on wp-config.php
3. I already tried changing the name of the plugins folder through the FTP
4. Of course I also tried your suggestion: adding error_reporting(E_ERROR); to the functions.php of the child theme.

Nothing helps. I just get a blank page.

Any ideas?