Solution:
I found a related discussion here: WordPress Support Thread
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The issue often comes from unwanted spaces or characters in functions.php.
Quoting Shane G’s advice:
- Download the file mentioned in the error message.
- Open it in a plain text editor.
- Ensure the very first characters in the file are .
- Place the cursor between ? and >, then press DELETE for at least 15 seconds.
- Type > again and save the file, making sure not to add extra spaces or lines.
If this resolves the issue, then the problem is with WordPress output starting too early.
⚠️ Sometimes, plugins that attempt to redirect from www.example.com to example.com can cause this error. Since headers cannot be sent once output has already begun, it’s better to handle such redirects in .htaccess instead of PHP. For example:
RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]