I've set up a VPN on my Mac mini using Server 4.1 with the "L2TP and PPTP" option set. I can connect to the VPN from my iPhone over my local network (Time Capsule) using L2TP but not PPTP. With the latter the VPN service log shows the error
DSAuth plugin: MPPE key required, but its retrieval failed.
I've Googled around the subject and found various answers saying my "VPN MPPE Key Access User" dismissing/corrupt and I need to recreate it using sudo vpnaddkeyagentuser /LDAPv3/127.0.0.1. These posts tend to point back to this article – https://support.apple.com/en-us/HT201828.
I've tried this, but I don't see the user being added to the list in Server (yes, I clicked on View > Show System Accounts).
Best Answer
Sorry for being untechnical, but I solved a VPN/Yosemite issue: After a smaller Upgrade my VPN still connected, but didn’t let anything thru. The Update somehow „unchecked“ System Preferences > Network > VPN(PPTP) > More…/Options > Connecting Option > Send all traffic through VPN Connection (third option in the first list in this dialog; I'm translating/guessing from the german version). Setting it back to let everything go through resolved all issues; Mail, afp, ftp, vnc came back just like that.