Cisco VPN client mysteriously stops working under Windows 8 -- symptoms:
- connection appears successful
- connection dialogue can still tell the difference between a right & wrong
  password, so authentication is working
- network stats shows packets being sent and encrypted successfully, but 0
  received / 0 decrypted
- enabling the debug log shows corrupted IKE packets received with "invalid
  version (random version number)"; messages like:
  Sev=Warning/3    IKE/0xE300002C
  ISAKMP header invalid: Invalid version 10.14 found

Solution: this happens when "Fast startup" is enabled (which does a partial
hibernate rather than a real shutdown / startup); if the VPN driver is
connected at shutdown, it gets confused next startup. Disable fast startup:
Power options > "change what closing the lid does" > enable options not
currently available (link at the top) > "enable fast startup" (checkbox at the

Then do a full shutdown and cold start.