As of this morning, I had a lot of trouble logging into codeforces, and wanted to put this up in case anyone else ran into a similar issue, and so that hopefully this can be fixed:
Trying to use username/pass login resulted in me seeing a 403 Forbidden Page, with the console yielding warnings like:
Cookie “JSESSIONID” will be soon rejected because it has the “SameSite” attribute set to “None” or an invalid value, without the “secure” attribute. To know more about the “SameSite“ attribute, read https://developer.mozilla.org/docs/Web/HTTP/Headers/Set-Cookie/SameSite
Thankfully the fix is relatively simple, changing the cookie state from Secure: false, to Secure: true allows login instead of receiving a 403 Forbidden message. This change can be done through the storage tab in the developer settings of either chrome or firefox.
Hope this helps someone, and hopefully this gets fixed soon so that cookies don't need to be manually edited to enable login!