I have the same message when I try to use encrypted user key for wifi connection (TLS, WPA & WPA2 Enterprise).
I found a mention about fresh same issue on https://bugs.launchpad.net/ubuntu/+source/network-manager-applet/+bug/1339607?comments=all, and one point of last (#16) comment can be a root of trouble. On my laptop I use full disk encryption LVM. So, I moved my protected key to mounted USB flash - and wifi connection works fine! I think, that problem in disk encryption. Can you approve this and fix it?
