verify_system_password() trusted the first loadable libcrypt. A library that cannot speak the shadow hash's algorithm (yescrypt, NixOS's default) answers with the '*0' sentinel, which was compared to the hash and reported as a wrong password -- even though the same password still signed in at the desktop. Following the System Passwords tile's 'sign out and back in' hint then locked the user out of the Hub, asking for a 'new' Hub password that does not exist: Hub login is the free account password, verified against /etc/shadow.
Try every loadable libcrypt, treat None/'*' replies as inconclusive rather than wrong, and when no library can decide, verify via PAM -- the same check GDM performs. Also replaces the sign-out hint (server + JS fallback) with wording that says the Hub has no separate password and not to sign out unless you know the current desktop password.
Recovery for an already locked-out user whose desktop login still works: on the machine itself, clear the localhost:8937 cookies and open http://localhost:8937/auto-login (localhost-only), or run 'sudo change-free-password' in a terminal to set a known free/Hub password.
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