HMRC have recently changed their specification to accept a wide range of non-alphanumeric characters, and a password length of greater than 12 characters.
Ftax are in the process of updating their forms to the same HMRC specification. We expect to have this work completed by April 2021, ready for the new tax year’s forms.
In the meantime, please ensure that your HMRC password contains only alphanumerics and is no longer than 12 characters. To change this, you need to use your HMRC account.