File upload problem from some systems

I migrated my app to a new server a few days ago and now i have a weird
problem. Uploading files (it worked before!) now works only for users
using Mac OS. When trying from windows it hangs with “sending request”.
The new systems is FreeBSD6.2 amd64 with the latest ruby (1.8.6-p111)
and rails install.
I tried with mongrel and webrick, it’s all the same.
Yes, the form enctype is correct, uploading does work for me and other
mac users.
I’m out of ideas after 2 days of trial-and-failure, can anyone suggest
anything?