I have an odd case but I think I know what the problem might be but I want some outside opinions.

I have a user that normally works from home and has no issues with the VPN connection. Today how ever he has gone to a customer location and is unable to connect. My first instinct is that he is behind a firewall that is blocking part of the pptp connection. See what it looks like in the log from my side of things below. I suspect if I switch him over to l2tp he might work or if I disable encryption it might work but I just want to get outside ideas before I do.
*edit*
User reported he tried using L2TP and still failing, did not see anything on this side of things.
*/edit*

In most cases,
the problem is the GRE - Protocol which is also ( TCP and GRE are used ) used for PPTP - Connections.
Often the ISP uses a DHCP - Server ( in a Router ) which leases non routeable IP - Adresses ( like 10.x.x.x ).
This works fine with UDP and TCP but makes lots of troubles with GRE ( not supported, blocked, no way back, ... )

So,
Your idea to switch over to L2TP is quiet good cause in this case, only UDP Port 1701 is required !

But,
don't forget to "allow accept input UDP Port 1701" in the Firewall - Rules !