If UNDECEMBER Mobile Login isn’t working, check the Android version, game update and sign-in account before changing anything. From 28 October 2026, UNDECEMBER requires Android 16 or higher. Android 15 and older devices no longer meet the live-service minimum, regardless of whether the app installed previously.
The October 22 connection issue has already been fixed, and the unrelated Google Play payment problem from October 28 was resolved. Check the live notice board for any new maintenance before assuming the servers are down.
UNDECEMBER Mobile Login: Check the OS and current app
- Check Settings, About Phone, Android Version. Android 16 or newer is required.
- Open Google Play and install any current UNDECEMBER update, including the compulsory update associated with October 30.
- Fully exit and restart the game, as LINE Games directed after the October 28 data patch.
- Check that Wi-Fi or mobile data is working normally.
- Look at the developer’s latest maintenance and known-issues announcements for an actual outage.
For UNDECEMBER Mobile Login errors, updating Android and the game comes before password recovery. A device still on Android 15 cannot connect to the current service, even with the correct credentials.
Why the wrong character appears after signing in
The game supports mobile login methods including Google, Apple, Facebook, LINE and the linked FLOOR account system. A different sign-in method can open a different account. Check the identity originally used for the character before creating another character or trying an account link.
LINE Games says accounts can share character progress across mobile and PC once correctly linked. But the linking procedure asks you to select which account data survives. Characters on the unselected account are permanently deleted during linking. Never press Confirm on an unfamiliar account-selection screen as an attempted login fix.
For UNDECEMBER Mobile Login confusion caused by multiple sign-in providers, use the original platform’s account-recovery process. Our Android and PC progress guide explains the irreversible selection warning before you connect any accounts.
How to check the server status
The October 22 connection issue was linked to a PC Shader 3.1 graphics setting, and LINE Games marked it resolved. Its earlier Windows launcher-folder workaround is not an Android troubleshooting step. Avoid PC file-deletion instructions when diagnosing your phone.
What if UNDECEMBER Mobile Login is stuck on authentication?
Record the exact on-screen wording and which login provider you selected. If the app remains open and reports a server or authentication error, check connectivity and any current official announcement. If it closes and returns to the Android home screen, you have an app crash instead.
Our Android crashing guide covers an app shutdown, while our download guide covers an unfinished installation. For UNDECEMBER Mobile Login failures, use the message to choose the right checks.
Report a sign-in error to LINE Games
LINE Games asks players to report bugs through its 1:1 in-game Customer Support, with screenshots or videos when possible. Provide your phone model, Android and app versions, server region, login provider and error text. If you cannot launch the game, use an official LINE Games support channel instead.
Do not share passwords, one-time sign-in codes or screenshots exposing recovery information. If your phone cannot get Android 16, our compatibility guide explains that unsupported OS versions cannot be fixed by switching login providers.
If UNDECEMBER Mobile Login still fails after those checks, report the exact message and login method. Keep your account connection unchanged until LINE Games confirms what needs correcting.
Related Guides
Official source: LINE Games’ completed October Android patch notice.
