Looking for logging into the Magento Checkout Not Working site and are not getting the correct working link for it? In such a case, you can use the links given here by searching your desired login page in the search box. If the page is present on our website then you will get all the links related to the Magento Checkout Not Working site. After that you can login as usual with your login credentials.
https://magento.stackexchange.com/questions/143539/user-registration-login-not-working-in-checkout-after-patch-8788
https://stackoverflow.com/questions/22483878/magento-customer-login-wont-work-on-onepage-checkout-after-1-8-1-update-from
https://community.magento.com/t5/Magento-1-x-Security-Patches/SUPEE-8788-on-Magento-1-7-causes-login-fail/td-p/51026
https://community.magento.com/t5/Magento-1-x-Technical-Issues/1-9-2-users-can-not-log-into-front-end/td-p/11958
https://community.magento.com/t5/Magento-2-x-Technical-Issues/existing-user-login-at-checkout-produce-error-quot-Provided-form/td-p/30201
https://community.magento.com/t5/Magento-1-x-Version-Upgrades/One-page-checkout-process-does-not-work-anymore-step-4-payment/td-p/16491