I didn't play with 3.4 that much so I'm probably not much help..
I wonder though if you can log on to the server via Remote Desktop and view the forum as a local user if it'll give you a better error message?
I know in 4.0 I can do that... but no idea how 3.4 handled errors... --
http://www.the-pork.net ... highly customized version of InstantForum.