Hi Giuliano C,
Not sure if it helps in your case, but I thought I'd mention it anyway. Have you tried or applied the workaround for setting the ReportPath within the HostedPageLoad event handler ? If not, please view the links on our ReportViewer page and see if it helps.
If this does not apply or doesn't help, I think I will need a simple reproducing sample application from you where I can reproduce. In that case, please create the most minimal reproducing sample application, clean it, zip it and then email it to the support email address (see my signature). Please include in that email the full Http reference to this thread and also please post here on this thread when you have sent it.
For this case, please include a simple report, preferably not databound. If it needs to be databound, please have it use the NorthWind sample database so I will be able to run the application.
Palli