VPN problems since Gateway firmware update

For several years now, I have been using a PPTP VPN to access my work from home via my UVerse connection.  This just uses the standard WIndows built-in PPTP client (Windows 7 and Windows 8 currently in use here).

Last Wednesday night my router was reset from ATT's end, to push new firmware, and since then I have been unable to establish the VPN.  I have had 5 chats with online support, and three sessions with ConnecTech (escalating to tier 3 there).  Nobody was able to resolve the issue or see anything wrong with the gateway configuration, so eventually they decided to send me a new gateway as mine was a bit old (3800HGV-B).

Yesterday the new gateway unit arrived (3801HGV), so I connected it, and as soon as internet connecivity was verified attempted to connect the VPN.  This worked!

Five minutes later the router was reset from the ATT end and had its firmware updated.  The VPN no longer works.  I take this as (VERY) strong evidence that the firmware update is implicated.  However, clearly this is not happening to everyone or there would be a rash of such complaints.

I spent several hours with my employer's IT admin going over VPN server logs, and trying various things.  Logs show I am able to connect , but that the authentication exchange is unable to complete.  Error messages at both ends suggest that the most likely cause is some device on the route not passing GRE through.  The 3801HGV admin is fairly locked down, and does not allow you to directly see which protocols it passes through (there is a list of protocol names, but neither GRE nor PPTP appear, however 'other protocols' IS enabled).

Confusing matters further - I have another VPN to a customer which DOES still work (from the same computer as the one to my employer that fails).  Its client configuration (apart from IP address of course) is identical to the one that does not (PPTP, max encryption, IPV4 only, ...).

We then installed WireShark to take a closer look at what was going on, but this didn't really clarify much.  The attempted connection to my employer fails prior to attempting any GRE activity.  The successful attempt to the customer shows GRE activity working just fine (so we can rule out any absolute block of GRE in the gateway router).

We then attempted a connection to another of my employer's offices, which uses a slightly different version of the VPN server (which is a Windows RAS server running on Windows Server 2008 at one office, Windows Server 2003 at the other).  It failed in the same manner.

I also configured the VPN on my laptop (Windows 7) and verified that it too failed the same way (it does).  However, if I take it down the street to a friend who has a different ISP (TWC), it works fine without any configuration changes.

So here is what we know:

1) PPTP VPN connections to either of my employer's offices fail over the ATT gateway with latest firmware (6.11.1.29-enh.tm)

2) The same connection suceeded with the previous firmware prior to last Wednesday on an older gateway (3800HGV-B).  I do not know what that firmware rev. was

3) The same connection suceeded with the factory-shipped firmware on the newer gateway (HGV3801HGV).  I didn't think to look at the firmware version in the 5 minutes I turned out to have before it self-destructed

4) Apparently identically configured VPNs do not fail to a customer VPN (server unknown, but same protocol)

Conclusion - there is some interaction between the latest firmware and something about the VPN, which looks like it must be server-dependent (not just protocol dependent)

Ideally I'd like ATT to wind back my firmware, as we seem to have run out of other options.  Does anyone have any suggestions to offer?