My Samsung is asking for a password when I don't have one

Well, I remember having some troubles with my password in the past where it wouldn't let me unlock it even though I knew the password. This was just a few minutes after I had been using it and my phone never left my hands, so it's not like anyone changed it to mess with me. In the end, I think I was able to say I forgot my password or something like that, and afterwards it removed my password and I was able to use my phone again.

But since then, whenever I try to set a new password, it always asks for my password even though I don't have one. I try putting in my old password with no luck. How do I fix this?