If you'll find the key error_invalid_form somewhere around line 84 and not in line 92 please upload the schript contact.inc.php from the 2.4.17-archive to the server again.

I do find it in line 92?

On a separate note I continued doing some testing of 2.4.17, I created a couple of dummy registrations with gmail, yahoo and mail.com accounts and still after a few hours not received the activation email for them, not even in the spam folders.