I added now the following lines:
return unless value.present?
return unless defined? record.id
return unless record.password_validation_required?
in additon to:
return unless record.should_validate_email_address?
As disco_bot is already created with an id on db.migrate this works now.
Furthermore, I had to add the password_validation_required validator, otherwise the plugin also fired when logged in and on changing the user avatar.
Now, it is really just bound to the sign_up form, and no seedfu filter required anymore.