Today I ran into this error when trying to start a Cisco AnyConnect VPN connection: Failed to initialize connection subsystem
After some reading I found the easiest way of fixing this issue is to use the Troubleshoot compaibitlity-wizard against vpnui.exe in C:\Program Files (x86)\Cisco\Cisco AnyConnect Secure Mobility Client.
Click Try recommended settings and note that the wizard has used Windows 8 compatibility mode:
Click Test the program... and apply the settings with Yes, save these settings for this program.
Restart the Cisco AnyConnect Secure Mobility Agent service or reboot your computer.
This happened on Windows 8.1 after I installed some updates, based on reports of other users I suspect Cumulative Security Update for Internet Explorer 11 for Windows 8.1 for x64-based Systems (KB3021952) may be the culprit. Other users report the same issue in Windows 10 Technical Preview.
Cisco AnyConnect observes the IE Work Offline setting for the System Account. Recent updates to IE changes the behaviour of IE which seem to have broken Cisco AnyConnect. Based on my research many similar product use the same approach and can run into issues on Windows 10 TP or older versions of Windows with update KB3021952 installed.
An alternative approach would be to create a key in the registry to fool Cisco AnyConnect. Create a new key GlobalUserOffline with a value of 1 under the path HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings.