There was a problem with the site - the FCK Rich Text Editor was returning a '404' error page within the frame where the editor should have been. I put up with this for a week or so as I tend to write straight Html in...but it was becoming a pain. So I investigated it and.... the whole /Providers/ path in this website had just gone completely missing. Once I re-uploaded it, everything started working perfectly. I think I'll never figure out how that one happened.
The problem also manifested in a 'Host Settings' page exception. I got a weird error which was something like 'path incorrect'. Of course, the Host Settings page looks for the DataProvider logs to show as the upgrade log. With no providers/ path, no log files, and a path error. Simple really.
So if you've been posting to the forums and getting annoyed with the lack of rich text editor - the problem is now fixed.
Now to get onto the mysterious failure which results in the 'install/underconstruction.htm' page...