Since I can't find a complete or clear answer on this issue, and since it's the second time that I use this post to fix my problems, I post my solution:
why 720? 720 is the error code for connection attempt fail, because your computer and the remote computer could not agree on PPP control protocol, I don't know exactly why it happens, but I think that is all about registry permission for installers and multiple miniport driver install made by vpn installers that are not properly programmed for win 8.1.
Solution:
check write permissions on registers
a. download a Process Monitor http://technet.microsoft.com/en-us//sysinternals/bb896645.aspx and run it
b. Use registry as target and set the filters to check witch registers aren't writable for netsh: "Process Name is 'netsh.exe'" and "result is 'ACCESS DENIED'", then get a command prompt with admin permissions and type netsh int ipv4 reset reset.log
c. for each registry key logged by the process monitor as not accessible, go to registers using regedit anche change these permissions to "complete access"
d. run the following command netsh int ipv6 reset reset.log and repeat step c)
unistall all not-working miniports
a. go to device managers (windows+x -> device manager)
b. for each not-working miniport (the ones with yellow mark): update driver -> show non-compatible driver -> select another driver (eg. generic broadband adapter)
c. unistall these not working devices
d. reboot your computer
e. Repeat steps a) - d) until you will not see any yellow mark on miniports
delete your vpn connection and create a new one.
that worked for me (2 times, one after my first vpn connection on win 8.1, then when I reinstalled a cisco client and tried to use windows vpn again)
references: