Guild Wars 2

When I log in I get a message to change my password, but when I type in my new password (which is 12 digits long) it gives me “Your password must be at least 8 characters long.”

First, there is a cooldown of 2 hours if you did wrong on the change.
Second, they check against a list of known passwords. It may be that the one you are trying is on that list. Somehow here I read that the error message might not very good.

If anything fails, you might ask support to change it for you.