UTM220 rev5 unable to change webadmin password

Dear all,

I have 2 old Sophos firewalls UTM220rev5 on which I'd managed to change the root and the loginuser passwords (following the instructions from https://support.sophos.com/support/s/article/KB-000034260?language=en_US). I'm not able to change the webadmin password (same link at step 16). I get the error displayed bellow.

Do you know how to tackle/solve this issue without reinstalling the entire thing (i.e. Linux, and all the Sophos software)? (I'd bought this firewalls from somebody)

Thank you!
R

PS: the error has the followings:
"
Calling Confd function system_password_reset()
result: 0
fatal: [{
  '0attrs' => [ 'class', 'type'],
  'attrs' => [ 'function'],
  'class' => 'ca',
  'fatal' => 1,
  'format' => 'The argument of the function %s must be a %_0 object and not empty.',
  'function' => 'ca_generate_host_key_cert',
  'msgtype' => 'ARGUMENT_EMPTY',
  'name' => 'The argument of the function ca_generate_host_key_cert must be a X590 certificate with private key object and not empty',
  'never_hide' => 0,
  'type' => 'host_key_cet'
  }]
"