Indeed scary, thanks for sharing

Is there any KB article telling how to debug this kind of situations (like where startup logs are being generated - event log or some file insidde SM dir) ? If not then maybe it would be good to have one with at least suggestions where to look for suggestions what could go wrong.

I had situation some years ago where I'm not sure how but domain has been added using API as "something@domain.com" instead of "domain.com". SM instance worked just fine until restart and then I could not start it. Emergency ticket was needed which obviously I had to buy. If I would know where to look for clues what could go wrong I could handle this situation on my own (simple XML entry removal would fix my problem).

Probably .NET Core version produces more logging in Event log than previous .NET Framework versions. Does mentioned JSON corruption was also there?

In Brian's case I ended up finding out the installer was failing to download pre-requisites (the ARR router and URL Rewrite components) and so while the install was ultimately successful, the necessary prerequisites were missing and so SmarterMail failed to present as you'd expect. The only signs that this was happening were 1) an error being displayed during the installer process and 2) the missing prerequisites per the readme in our installer and so it did take a little bit of poking around to confirm that was what was happening. I'm digging into why Brian's environment is having issues reaching those pieces and will be discussing measures to prevent this in the future with development. 

But a separate installation program with all the prerequisites, with all the checks that everything has installed correctly?

Ron, that’s what I did for my business server. Everything I did to fix the home server, I did preemptively to the work server. Just waiting on a couple answers re .well-known and BIMI prior to proceeding. 

If the ARR URL was offline for a reason then the installation would fail.

The server resolves to everything else just fine so the problem is maybe temporary.

I suggested to Kyle that ARR installer should be a part of the package so these things wouldnt happen.