Some ideas to troubleshoot this:
- Create your password in a text file and save it there. Then copy and paste the password here.
- Try all Latin characters (A-Z)?
- Let your web browser save your password for you.
- Try a different web browser.
- Test the same thing on xenforo's forums to see if you have the same problem there.
I hope something there works and if you find out what the problem was, let me know so I an help others with the same problem.