Unusual postback problem

I'm having an unusual problem with an ASP.NET 2.0 website. The problem happens on some form button submissions only on one particular server. The page works fine when hosted on other servers. So, on the problem server, the postbacks from my buttons take place ok-- they are triggered, but the server throws a 504 level error. The site does process the postback, however, proven by storing the expected submitted form data in a database. So, something goes haywire when trying to process the response it seems. (The server is not in my control so I can't see the details of the 500 level error.) But here's the clincher: If I happen to be logged in to the server with my Juniper SSL VPN account, everything works great. If I log out again, the forms continue to work, until I close my browser. Then it's back to the failures. Ah ha! Sounds like a cookie issue. But that's weird. The program doesn't rely on any cookie usage that I put in there, and I don't use the auth ...