ASP.NET FileUpload.HasFile = false when trace disabled

So, I have an WinXP SP3 development environment running ASP.NET VB Website 3.5 SP1 and something weird (at least to me) is happening:

I put a FileUpload inside a page and it isn't 开发者_运维技巧inside any UpdatePanel (it has a script manager in the MasterPage, though). The fact is that when I disable the trace in Web.config it doesn't upload the selected file, bug guess it when I enable trace it works.

Any suggestions?

Best regards, Marco Alves.