Sometimes WordPress issues could be caused by the simplest of things. This is especially true for headers already sent by WordPress error; this error can lead to the inability to login from a user dashboard and, hence, it prevents you from managing the site properly.
The issue usually manifests in a blank page and, in case WP_DEBUG is set to true (more on this bellow), in an error log pointing to the headers already sent by error, a file path and the exact line which caused this error.
So you might see something like this displayed on your screen:
Warning: Cannot modify header information - headers already sent by (output started at /some/file.php:12) in /some/file-2.php on line X .
Reasons for this error could be multiple, but the most common one are the whitespaces, either before <?PHP or after the ?> part of the code.
Troubleshooting:
Step 1 - First off you should access the error log. This can be done through FTP, via an FTP client like FileZilla. Find the wp-config.php file and set WP_DEBUG on true.
Step 2 - Go to your login page, which is often your-site-url/wp-admin, and review the error message displayed.
Example could be as follows:
Warning: Cannot modify header information - headers already sent by (output started at \wp-content\themes\theme_name\framework\modules\woocommerce\widgets\woocommerce-dropdown-cart\woocommerce-dropdown-cart.php:121) in /wp-includes/pluggable.php on line 922
This means you need to navigate to this path, via FTP:
\wp-content\themes\theme_name\framework\modules\woocommerce\widgets\woocommerce-dropdown-cart
and locate the said 121st line of code in that file.
Removing a blank space before or after this line in the specified file will in most cases solve the problem.
Step 3 - After the changes are done, save the file and use it to override the existing one.
In case you don't feel comfortable performing these changes yourself, you can always submit a ticket explaining the issue and let our support team assist you. In this case, please include your FTP credentials in your ticket.