Looking through the pinned posts in this forum I've come to the conclusion that my account was either mini banned or my account got hacked, given that I'm getting the "The information you have entered is not valid" screen while trying to login. However when speaking to my in-game friends they told me no-one has yet tried to access my characters, at least.
I have tried waiting for 15 minutes between login attempts but to no avail, and have also tried resetting my password at https://en.nostalrius.org/password, but keep getting the error message "Account with this email not found".
I've tested everything related to my issue in the pinned posts.
Thanks in advance!
Edit: it started when I was disconnected in the middle of a session playing a character, and in quick succession repeatedly tried logging on again.
Edit2: Never mind, I am a silly, silly man. Turns out I was trying to log in using the wrong username.