Stefano, trust me on this one .. I guarantee you it's a server side configuration error. :)

My fault though, the command you need to run is aspnet_regiis -c 

The problem is usually caused by installing Service Pack 1 of the .net framework. The issue is that the aspnet_client folder is not where the .net framework is looking for it. running aspnet_regiis with the -c parameter will re-establish the client-side javascript location. When installing SP1, it usually generates an error that is hidden in the log that says "Failed while copying the ASP.NET client side script files". So you most likely still have the old pre-sp1 WebUIValidation.js file in the aspnet_client folder.

This is not a new problem. I have had this problem myselft and other have reported it as well

Stefano,

I've had to run that regiis fix myself several times - be sure to clear all client and server caches as well.  But since Opera works on the production site, my suggestion would be to make sure that IE is not disabling JavaScript for the remote site?  This is in Tools, Internet Options, on the Security tab.  See what the settings for scripting are if you are using a custom level, or for simplicity/speed, try adding your production site to the "Trusted Sites".

If you're OK with it, have a look at the page source and compare your production page to the local page just to make sure the js code is in there, so you're comparing apples to apples.  Let us know?

--
Roberta