Are you using a password manager and/or browser autofill? We had another person running into this issue recently and it turned out their password was longer than lemmy.world accepts, so what happened was that it got cut off as they were registering and took a shortened form of the password instead, but without indicating that.
See the thread here for some details. TL;DR is that it appears the max length of a password is 60 characters, so if your password exceeds that and you have a tool autofilling for you (such as your browser), this may be what's preventing login.