Found: After a restart back to character select without closing the game, the server still remembered the FireGuard password as entered, so when the game asked again it refused the correct password as "already verified".
Fix: Returning to character select now resets FireGuard, so the game asks once and accepts the correct password; the correct password entered again is also accepted. This is in the next RF Online 4.55 update.
To verify: Log in, enter the FireGuard password, use the in-game restart to go back to character select, and enter the password again when asked: it should be accepted.