My admin is broken or login loops after the upgrade. How do you fix it?

An admin that loops or 500s after an upgrade is usually a stale compiled generated/ directory, a broken admin theme or layout XML, a session/cookie domain mismatch, or a third-party admin module that did not survive the version jump. We clear and recompile the generated code, flush cache, check the admin URL and cookie config, and disable or bridge any admin module that is throwing. You get the working admin back plus a note on what caused it.