WPToolkit Login returns "Internal Server Error: Header values must be RFC 7230 compatible strings"

Symptoms

When clicking "Login" on WPToolkit, you get the message "Header values must be RFC 7230 compatible strings." This error also shows up in the WP Toolkit mainlog. Additionally, local application error_logs will show "PHP Deprecated" errors similar to this:

CONFIG_TEXT: [01-Jan-20XX 00:00:00 UTC] PHP Deprecated: Elementor\Modules\GlobalClasses\Atomic_Global_Styles::get_cache_root_key(): Implicitly marking parameter $key as nullable is deprecated, the explicit nullable type must be used instead in /home/mehtosec/public_html/wp-content/plugins/elementor/modules/global-classes/atomic-global-styles.php on line 410

Cause

This error occurs when the script or plugins are running on the incorrect version of PHP. The local application error_log will often highlight which plugin is generating the error.

Resolution

Downgrading or otherwise changing the PHP version to the properly supported version will resolve this issue. You can review the instructions here for changing PHP versions for a domain:

How to change the PHP version for a domain in WHM