Accounts :: Unable To Login As Standard User On Windows 8.1 PC

My son is no longer able to logon to his standard user account on my Windows 8.1 U1. I am unable to switch users following a cold start of the computer. Also, I am unable to switch users while logged on to my account.

My System:
Dell XPS 8100 with Intel i5 processor and 6GB DDR3 ram. The desktop initially included Windows 7 Home Premium. I upgraded to Windows 8 Pro as a $40 Windows Store purchase. I've since updated to Version 8.1 and the first update following the release of 8.1. At time of upgrade to Windows 8, all prior updates to Windows 7 were current and successful. Since upgrading to Windows 8, all updates are current and I only experienced 1 update "Failed" (KB2919355), which upon a reinitialization after the failure, was successful. Until this problem, the PC and operating system have run very well with no real problems.

As noted above, there are 2 users on this PC. Myself (administrator), and son (standard user). I can't pinpoint an exact date when my son was no longer able to logon to his account; however in checking the registry I find that the last notation of his login activity under error reporting was a successful login and logoff done on April 30th. And even though attempts have been made since that time to login to his account, there are no notations of this in the error reporting log that I can find. Most likely that's because I can't get to a point that will allow me to "initiate" a login for his account. It's as though his account does not exist.

What Happens When:
Boot - When the computer boots, it only presents a login screen for my account. There is no right-arrow allowing me to switch users. Switching to a secure login via Ctrl Alt Del yields the same results.

After I'm logged in - If I click to switch users, once again I will get a login screen with my account only and no way of switching users. If I wake the computer from sleep or from screen saver mode, I'm presented with my user picture and password, however, this time I do get a right-arrow to switch user. When I click the arrow, the screen goes black for about 5-7 seconds, at which time I'm presented my user login and no switch user arrow.

Efforts to Resolve:
I've researched this issue the the past month or so and have checked and/or tried just about everything that's out there from Microsoft, Windows 8 Forum, The Windows Club, and a few other sites as well. As I follow the suggested fixes, everything seems to be as it should be. I've yet to find anything that does not appear to be "right". For example: SFC /scannow says everything's OK. If I go to User Accounts, both accounts are present. If I go to C:Usersmy son's account, it is there. I am able to create another account with name and password. However, I am unable to open that account for the same reasons that I'm not able to open my son's account. The new account created is NOT reflected under C:UsersNew Account, so I'm unable to copy files from my son's account to the new account.

Because of the time delay from when my son first informed me that he's been unable to open his account, my restore points are all following when it probably occurred. I do have a Macrium image from late March that is available. However, I'm wondering if I brought the PC back to a state from this past March if I would be able to install all updates to get the system current. I know Microsoft was imposing time constraints associated with updating to Update 1 following Windows 8.1 ...