🇺🇸United States @ronalpha

form_id was returning null or empty when executing form validation logic.
This code _antibot_generate_key($form['#form_id']); in antibot.module

Changed to
_antibot_generate_key($form_state->getValue('form_id'));
Now the form_id is returning the correct value and the form is processed with no antibot related errors.

@chdeepak @ anand.panure... this is related to simplesamlphp/saml2 changes on 4.6.11. https://github.com/simplesamlphp/saml2/compare/v4.6.10...v4.6.11. Try going back to 4.6.10

Error: Cannot access property starting with "\0". ... issue appears to be related to the update of simplesamlphp/saml2 to 4.6.11. See https://github.com/simplesamlphp/saml2/compare/v4.6.10...v4.6.11. Try going to 4.6.10.