[Tim Wood]

I've added some configurable debug logging that will show up in v48.0.94931
(I just committed it, so it'll be a while before it shows up in the download list).

You should be able to do the following in Terminal:

defaults write com.omnigroup.OmniFocus OSUDebugDownload -bool YES

Then, launch OmniFocus and do a software update. There should be some logging
in /Applications/Utilities/Console.app that will maybe provide a clue as to where
this is going wrong.

Once you've gathered the logs, you can do:

defaults remove com.omnigroup.OmniFocus OSUDebugDownload

to make the logging stop.