Hi @austyfrosty . Unfortunatly, my site got another fatal error. The fix change the original fatal for another not fixed syntax.
PHP Fatal error: Uncaught Error: Typed property class@anonymous::$queryCacheKey must not be accessed before initialization in /wp-content/plugins/custom-login/vendor/thefrosty/wp-utilities/src/Api/WpCacheTrait.p
hp:37
I am getting exactly the same error than @ernestph very often. If it occurs once, this is going to continue throwing always, unless making public the $wpRemote->retrieveBodyCached method and removing $wpRemote->getQueryCacheKey(), which throws a error due to not initialized property.
My site runs in PHP 7.4. It does not seem a conflict with another plugin.
@austyfrosty You can see reproduce the error in wp-admin/options-general.php?page=custom-login ang get the screen: