410 Gone error if I insert "shell_exec" or "$_SERVER" inside an HTML input form

Question

I have this very simple web HTML form on my web hosting:

<form method='post' name='myForm' action='saveForm.php' enctype='multipart/form-data'>
<input type='text' name='title' value="" placeholder='title' size='50'>
<input type='file' name='newFile'>
<input type='submit' name='saveSubmit' value='Save'>
</form>

If I insert the string shell_exec or $_SERVER into the input text (with or without other text before or after), the 410 Gone error occurs immediately after submitting the form. It happens on my PC with a fiber cable connection and also on mobile with LTE 4G.

I have already checked that my php script on the server is not reached at all, because its very first lines output nothing:

<?php
var_dump($_POST);
exit;
...

After it happens, all other requests to my site result in waiting a response for at least 3 minutes ending with the "connection time out" in the browser.

If I try on mobile with LTE, so with another internet connection, the site is correctly immediately displayed. Because of this I should exclude that the problem is web hosting related.

I already tried these without success:

• reboot pc
• change browser, also in privacy mode
• completely clear browser cache and all others data
• reboot modem
• wget from terminal -> same waiting time as in the browser

How can I figure out the cause/problem?

Ps. tell me if I posted my question on the wrong stackexchange site.

Thanks