My first thought would be that either the two users are not part of the same role center, or you’re not opening the same database. Second thought is that you have a customization gone bad. At this moment, I don’t have a specific example in mind that would raise this error (although I would expect that an errant field reference would raise a compile error before it would raise a runtime error).
You might start by compiling your most-recently modified pages, and then restart your NAV database services, and then try the connection again. Failing any definitive results from that effort, you might need to deploy the debugger.
… and I offer this as an afterthought. Can you confirm that no one has made any alterations to any of the table structures outside of the NAV development environment, say possibly by using SSMS?
And Rishi, have you yet tried running the problem code with the debugger? If you have, were you able to locate the problem code? If you haven’t, that would be my next step.
I’m not really sure what that means, but if you’re interested, you can google Dynamics NAV 2009 Debugger and get all kinds of blog posts on how to use it.