VPN (PPTP) Failure After Upgrading to Windows 10

Question

My small office recently upgraded every workstation on our P2P network to Windows 10 Pro. Since the upgrade, our telecommuter has been unable to connect to our VPN server using the Windows-based VPN client. She had no problem connecting before the upgrade. The error message received is as follows:

The network connection between your computer and the VPN server was interrupted. This can be caused by a problem in the VPN transmission and is commonly the result of internet latency or simply that the VPN server has reached capacity. Please try to reconnect to the VPN server. If this problem persists, contact the VPN administrator and analyze quality of network connectivity.

Here are some details on what I've tried so far:

1. Ensured that the firewalls on client/server allow bi-directional traffic for PPTP and GRE. This didn't work, so I disabled both firewalls temporarily and tried again. Still no connection.

2. Ensured that VPN passthrough as well as port forwarding for 1723 is enabled on our router and pointed to the server (we do not have a true VPN interface on our router).

3. Configured rules for incoming connections on the server to match our needs, i.e., appropriate user given permission. I tried specifying an IP address range, letting DHCP do the work, and letting the client specify its own IP address. None of these worked.

4. Updated WAN Miniport drivers (IPv4, IPv6, PPTP) in Device Manager.

5. Reset RRAS on both client and server.

Nothing has worked so far, and I'm out of ideas as to what the problem could be. How can I diagnose/fix this?