VPN is killed whenever branch office users log onto Windows Domain

I have posted sequential questions on EE about this in the past week, with each incrementally improving my situation. Here's the latest: I have a VPN running through a Sonicwall TZ170 and Netopia 3347 Router at my branch office. When the VPN is up and running users can access Exchange, server folders and files, and the Internet. The moment anyone logs onto the Domain from their workstation, the VPN crashes. I have worked with tier-2 AT&T technicians and had them cap the line speed down to 2.5MB, after trying 3.0 and 6.0 respectively. No matter what we do, any domain logon from any workstation in that office will absolutely blow up the VPN tunnel. As soon as I reset the Sonicwall the tunnel comes right back up and works fine. I have four offices, including this one, using identical configurations with TZ170's and they work fine, domain logons included. This is totally off the wall and I cannot find an answer to this.
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try replacing the router , or at least checking that it is the same firmware as the others.

Even better, switch a working with a non-working router, so that you can determine if it is router or location dependent.

I hope this helps !

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