I've tried this several times, following instructions at https://our.umbraco.org/documentation/getting-started/setup/upgrading/general (which, I have to say, aren't very good). Anyway, as far as I can tell everything was done correctly. Config file merging was long winded and tedious, but I'm pretty sure I got everything (went through each file very carefully with WinMerge).

When I finally run the site, I can log in and then get the happy-crowd 'Upgrading Umbraco' page. When I press the green 'Continue' button I get (either immediately or after a few Umbraco 'Did you know…' messages) "Error during installation. See the log for full details…"

But the log at App_Data\Logs doesn't seem to contain any clues at all!

I've been trying to accomplish this, on and off, for weeks now and it's becoming a problem.

Thanks