Cannot enable guest account
As per David solution here (although not the cause):

I used Directory Utility (/System/Library/Core Services/Directory Utility) to delete two extra Guest accounts (&quot;Guest1&quot; and &quot;Guest2&quot;).
After deleting those two records I could open the Users &amp; Groups System Preference Pane.

Source: I've got &quot;Preference Error: Could not load Users