From https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/518480

"Approximately 40% of the times where I boot my machine and load my Desktop, Network Manager throws up a password box that says "A password is required to connect to "Vodafone Contract" HUAWEI Technologies HUAWEI Mobile" (see screenshot). However, it does not accept my password. There appears to be no pattern to this. This happens when no changes have been made to my system - no recently installed updates etc. On one occasion it didn't happen for as long as five days, on another occasion it happened on four reboots in a row - it really does seem completely random. The only option is to open network-manager-applet, delete my mobile broadband connection settings, then run the connection wizard again to re-apply them from the beginning."

I've experienced this issue myself, NetworkManager asks me for a password and if I simply press enter with a blank field, eventually it will work after doing so a few times.