IE issue: does not react because of long-running script
This seems to be an IE issue. When endusers login some experience strange behavior in IE. The message "<site> does not react because of long-running script" is the most specific that we have heard off until now. What can cause this error? How can I find out which script? Could it be caused by theming? Update: I can now reproduce hanging within IE (9.0.8 64bits) on our own site. Works fine in firefox and chrome. I'd very much like to hear how I can best debug this. Update2: Starting fresh from the workshop stylesheet seems to have done the trick. I guess the old one somehow was not clean.
Frans Verschoor (FraternIT)
Open the developer tools (F12), start the script debugger and wait until an (uncatched) exception occurs, and keep an eye on the script console for errors.
Adding to Michel's answer, I suggest making sure all your widgets are up to date.
Are there any widgets used on the first page they see? Try removing these to test it if one of those is causing it.