Werk #18995: Fix Unregistered YubiKey Authentication Crash

Component	Core & setup
Title	Fix Unregistered YubiKey Authentication Crash
Date	Feb 24, 2026
Level	Trivial Change
Class	New Feature
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.6.0b1 Not yet released Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Ultimate MT 2.5.0b1 Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Ultimate MT 2.4.0p23 Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Ultimate MT

Fixed an issue where the authentication process would crash in certain circumstances if an unregistered YubiKey was used for two-factor authentication (2FA).

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