为什么我在发布带有“from”字样的文本时会收到错误500？

Question

I have this CRM that I built and the user can enter notes, but one of the notes won't save, returning an error 500. I checked the error log and there is no record of that error. The process is simple, it is a AJAX request that get's the text from a textarea and sends it to a PHP code to save the note into a MySql.

The funny thing is that if change the word "from" to "fro m" (with a space) it works. But if I remove the space, I get the error 500 again and no log information.

Also if I try it multiple times my IP gets blocked by my server's Firewall.

This is the note:

5/9/2019 Territory: ORIGIN: AB CITIES selected by the customer specifically DESTINATION: AB Cities selected specifically by customer. DO NOT EXCEPT any out of territory disputes from this account unless the city is not listed. He knows he can call in to adjust territory but will not receive credit for any cities he is removing. Lead Price: $6.00 USD Recurring: $200.00 Initial Charge: $300.00 USD. Customer understands the policy. TEST lead has been sent to him. Waiting for ED to work on final cities for origin. JH

$.ajax({
         url: "scripts/provider_note_add.php",
         type: "POST",
         data: 'provider_note='  + $("#provider_note").val(),
         success: function( data ) {
                  $("#result_notes").html(data).fadeTo(500, 1);
         }
});