L2TP error sending Hello (Can't assign requested address)
It appears other users are also noticing the issue https://discussions.apple.com/thread/6605525 


  When using a static IP address and manual entries for subnet and
  router in the network settings, Yosemite VPN fails to stay connected. 
  This is probably a bug or an error that Apple needs to fix.
  
  When having the client computer (the one you are using) network settings
  configured to either just use DHCP or DHCP with manual address, the
  VPN connections (both L2TP and PPTP) work.  If using ARD to access
  computers on the local network, the results are better if you select
  "Send all traffic over VPN connection" under advanced options for the
  VPN connection.


I just switched to OpenVPN to work around this issue.