Two data points:

(1) I have the impression that if you use the scroll wheel on the mouse, you can still navigate most of the time. The error message box stays there but you can scroll the thread behind it.

(2) Got a new error mssage just now, telling me that Shockwave Flash had stopped operating. I clicked on Stop script and carried on.... Is this what's behind it all?

I've just updated it (and avoided installing the ghastly Norton in the process), and will see if that changes anything.

Edit: And I am also noticing consistently that if you click to edit a post it spools for ever until you click the Edit button a second time.