I was moving a client's site from under my website to their own domain, same server. Copied the files over, got the site to show up. However, a few problems emerged. When I upload the new wp-config file that lists the new database location, I get this error message: Warning: Cannot modify header information - headers already sent by (output started at /home/watsonbk/public_html/wp-config.php:1) in /home/watsonbk/public_html/wp-includes/pluggable.php on line 866.
At this point I can not access the site or the dashboard. When I go back to the original wp-config file that goes to the database under my website, the site shows up, but the inside pages have a 404 error. I think I may be able to fix that, but the first error message throws me. I made no changes to the pluggable.php file, so why does that one suddenly have an error?