Ian,

Well...the attempt to go from 4.3.4 to 4.6.0 was a disaster. 

I got the "Value cannot be null", "Parameter name: type" error.  I checked the database.  Assuming that the upgrade process won't update the Versions table with the latest version until AFTER the database changes had been successfully applied, the database was updated properly.  Most threads I found on this error seem to point to either  changes to App_Code or cleaning up unused modules. 

Using Beyond Compare (thanks for the recommendation Arash...I've been using BC for over 5 years and it's one of the utilities on the market) I cleaned up the dnn folders (most of it crap from the aborted 4.8.4 install) and ended up with the same result that I got when I attempted the 4.8.4 upgrade -- a blank page. No errors, no messages, nada, zilch.  Nothing in the eventlog or sitelog tables and nothing in my IIS logs.

I deleted ALL dnn folders, reloaded them by hand using FTP from my backup, got the ISP to restore the database and, at least, I'm back where I started.

Unless you or anyone have any better suggestions, I'm going to take this one minor version at a time, starting with 4.3.7, then 4.4.1, and go from there.

I will say that DNN seems to be lacking serious logging support.  I know it logs stuff in the database tables...but what if the db support is broken?  A rolling text file logger would be a great enhancement.

Dave