My son has an account on our 2013 Mac mini running El Capitan, guarded by Parental Controls. Every few days or so, he has a login failure that causes his login to hang on exactly the second attempt.
Most logins work as normal. However, if we're going to have a bad day, the first login appears normal until about ten seconds after I authorize his time with my login. The screen turns black with a still visible mouse pointer, and about ten seconds after that, he's kicked back to the login screen. The second login thereafter (which doesn't require parental authorization) gives the rainbow beachball for at least ten minutes, at which point I just reboot the machine.
No other account on the machine is affected, so I assume he's done something or installed something that's causing this. It's also possible that Parental Controls are wonky.
The problem seems to be caused (or, at least fixed) by his Preferences; I delete everything in
~/Library/Preferences/*.plist and he can log in again.
I've actually gone so far as to recreate his account several times, and, annoyingly, the problem reappears after a while.
What can I do to debug this and get to the root cause? I've been through system logs and kernel logs, but there are literally hundreds of various "error" messages, all of which seem to be being thrown by any user logged in and aren't unique to his account.