After entering your password, try pressing the little 'eye' icon to the right of the password field to make your password visible (!). Then click 'sign in'.

Don't know why, but since the site redesign, that's the only way I can log in. Not that the site works properly - it claims I have no upcoming stays, when actually I have one upcoming reward stay. Haven't tried booking on the new site yet so can't confirm whether it errors out for me as other people are reporting.

If it's an option for you, you may want to consider using the mobile app until they get their website fixed.