VPN (PPTP) disconnects in OS X El Capitan (after update)
I've been struggling with VPN since 10.11.4. I'm unable to connect to a VPN server that I've used for a long time. I hope this is not noise, but here's what I have encountered in my research...

My problem is with an L2TP tunnel. https://discussions.apple.com/thread/7502488 discusses a potentially related problem with respect to Cisco IPSec VPN. The interesting part there is that some have reported success changing to Diffie-Hellman group 14.

Slightly related is the App Store description of the 10.11.4 OS X server where they say "L2TP/IPsec VPN profiles now require 2048-bit or stronger Diffie-Hellman groups". This is consistent with the group 14 reports above since group 14 has 2048 bits.

At https://supportforums.cisco.com/document/12276506/diffie-hellman-groups they discuss the relative strengths of the various groups. One post says that group 14 is the minimum acceptable security for key exchange.

So I've started to think that Apple is now ruling out groups 1, 2, and 5. This is bad for me since those are the only groups supported by the older tunnel server I used to connect to.

I also use Private Internet Access sometimes. I tested connecting to them using L2TP. It worked fine. However, even with verbose logging, I couldn't find any log entries related to a negotiated DH group. But PIA is a security focused service. So, I'm guessing they would avoid a DH group considered insecure by some.

I contacted Apple Care. They were unable to help. I didn't push them to escalate it.

I tested from my Mac using wireless, Ethernet, and tethered to my 3G phone. My work partner, running an older mac that hasn't been updated to 10.11.4, connects fine to the server.

I also tested with my iPhone. It cannot connect either. Unfortunately, I've updated that recently to iOS 9.3. forums_developer_apple_com/thread/31608 reports problems with iOS 9.3 in addition to OS X 10.11.4. (Sorry, I don't have enough reputation to post that third link.)

Regards, 
Barry