The error from debug seems to have something to do with hcaptcha, which I have enabled.
PHP Fatal error: Uncaught Error: Typed property ConstantContact_hCaptcha::$language must not be accessed before initialization
in /wp-content/plugins/constant-contact-forms/includes/class-hcaptcha.php:167
Stack trace shows it triggers during wp_enqueue_scripts → wp_head on front-end page load.
Yes, it was a critical error – “There has been a critical error on this site”. And, yes, I tried a second time to reauthenticate and as soon as I clicked Save on the code from Constant Contact, the site went down again with the same error again. As soon as I deactivated the plugin, the site came right back up.