This appears to be a duplicate of this post, but the solution there didn't work for me. Clearing the credential storage wasn't even an option -- it's greyed out.
Starting at the beginning -- I started to set up a VPN on my phone (Note 3, Android 5.0) but cancelled the operation halfway through, yet the OS still enforced the security settings (a PIN). I have tried a few things trying to disable the PIN lock, but the only option I have is to upgrade the security to a "password" which is even more unacceptable.
So far, I've tried finishing the VPN setup (it worked) then removing it, also force stop/disable of the VPN app, and quite a few other things that I can't recall at this moment.
I think part of the issue is that a certificate was installed when I started the process, but I have no idea which certificate it was. The System cert list is populated with tons of entries, but the User cert list is completely empty. Is there a way to determine which certificate was installed so I can remove/disable that one? I'm concerned with choosing the wrong one, as who knows what it might break on the phone.