Q: iPhone 6 bug?

Basically, my problem is that I cannot sign-in to iTunes nor iCloud using my apple ID on my iPhone 6. The error message that I get is that the password that I enter is incorrect. After trying so many times my account got locked. So I resat my password. Only thing is that even after resetting my password, I still cannot login to my account because of "incorrect account or password". However, by using the iTunes on my computer, I can login to my account without any problem. To be more certain, I logged into my account using a different iPhone and the login was successful. So after this, I got suspicious that the issue might be with my iPhone, so I signed in using a different apple ID on my iPhone and the sign-in was successful. I switched off my iPhone and resat it but nothing worked. I can't figure out the issue, is it a bug? Is anyone familiar with this issue?