From the official Discord last night:

Dear Players,

Please be advised that there is currently an issue with the login servers and you will get a 'incorrect username or password' error. The team is working on fixing it and we will notify everyone here once it has been resolved. We deeply apologise for this situation and thank you for your patience. Best

Regards, Naruto Online Support Team

The issue should be resolved now. We advise staying part of the official Discord for any in the moment notifications:

https://discord.gg/narutoen