PIN sign-in problem solved - at least, for this issue.
Windows 8.1 Pro with latest updates.
The problem:
Unable to use PIN sign-in as alternative to the full Microsoft Account password
(PIN is particularly useful when resuming from short screensaver timeouts eg during phone calls)
What one sees:
in [Charms] Settings / Change PC settings / Accounts / Sign-in options
'Picture password' 'Pin' and 'Password policy' are all
greyed out and inactive.
PIN is therefore not offered as an option when signing in.
Solution:
in Control Panel\User Accounts and Family Safety\User Accounts
(page entitled 'Make changes to your user account')
select 'Reset Security Policies'
Note:
This option is the last one on the list on the left of the
User Accountspage
(below 'Change my environment variables')
Naturally that will not work if the 'Reset Security Policies' option is not displayed or
if other actions are required, like enabling PIN password in Local Computer Policy {gpedit.msc},
Credential Manager Service etc.
Result:
Having clicked on 'Reset Security Policies'
I went straight back to Settings... Sign-in options
'Pin' etc were no longer greyed out, and worked as they should.