For me, it was that I had set up VPN. I simply deleted my "VPN network", and all of the unlock options returned. The error message seriously needs to be made more clear.
Also, while searching for the answer to this problem, I noticed SCORES of totally odd, some "Turkish", some "Russian", "Trusted credentials" under "Security"! WTF?!?! Why am I "trusting" so many totally unknown sources? This is alarming--though perhaps "normal" and "harmless". I am tempted to, now that the option has also reappeared, "Clear credentials", but the warning that pops up is "Delete all content?"! I do not want to delete all of my apps and data... just the cruft. Well, I did it, and it looks like all my apps and data are still in tact, alas, so are the "Trusted credentials".
PS: Holy !@#$! It is nefarious! DISABLE T?RKTRUST immediately! Read up on this:
http://ryanzag.blogspot.com/2013/01/why-googlecom-fake-certificates-are-big.html