Hello, I also have the problem.
Here is a solution (not the best):
/***/wp-content/plugins/very-simple-contact-form # diff vscf.php vscf_fix.php
140c140,144
< $server = preg_replace("/[^a-zA-Z0-9]/", "", $_SERVER['SERVER_NAME']);
---
> if (isset($_SERVER["SERVER_NAME"])) {
> $server = preg_replace("/[^a-zA-Z0-9]/", "", $_SERVER['SERVER_NAME']);
> } else {
> $server = 'CRONTAB';
> }
The variable ‘$_SERVER[“SERVER_NAME”]’ is not defined when running the WordPress crontab (wp-cron.php).
The email I received from crontab (every 15min): PHP Warning: Undefined array key “SERVER_NAME” in /***/wp-content/plugins/very-simple-contact-form/vscf.php on line 140