EDIT: I was able to solve this by going into the "change password" screen, right-clicking the "old password" field, clicking "inspect", and changing maxlength="60" minlength="10" to maxlength="60" minlength="1", thanks to the tip provided by Dandroid in the comments.
When I try to login, the following message appears: "Please use at least 10 characters (you are currently using # characters)." Ditto when I try to change my password.
This issue affects me when trying to log in from Firefox and Chromium, in Linux. When trying to log in from Firefox in Android, I can't but no message is given. It does not affect Jerboa or Voyager, but I can't change my password from either.
Any idea on how to solve this? When I created this account 2y ago I was just checking Lemmy out, so I didn't bother with a strong password back then, but this has become a ticking bomb. I'm currently able to access Lemmy from Firefox due to saved credentials, but I'm worried about them eventually expiring.
Pictures showing the issue:
(My actual password isn't 6 chars long, but the error message is the same.)