Many people use the same password for multiple sites. This could create a potential hole for a hacker to get your info which is what we would like to avoid. The forced password change for now is to insure there are no holes on the forum and the one that should happen a year from now can be revisited then and we can look into if it is still needed on the site.
Copy/Paste the password (as plain text) and make sure there are no space on the sides when transferring it from the email to the site.
If the issue still persists, you can use the Contact Us, or PM a mod/admin directly on the site and it will be resolved.