Thanks for the quick reply as usual!
I did try that: Fresh instance install, start instance, then "update". Unfortunately, it didn't work.
However, I think I see what the problem might be. When I looked at the steamapp logs I found the follow error:
Log Location: C:\AMPDatastore\Instances\Factorio01\factorio\logs\content_log.txt
[2019-11-24 19:26:43] Loaded 0 apps from install folder
[2019-11-24 19:26:49] Loaded 0 apps from install folder
[2019-11-24 19:26:49] Failed installing AppID 427520 (No subscription)
So just for kicks, I tried I tired using the steamapp cmdline to install Factorio using my own steam account and it worked. Unfortunately, when I started the game with AMP it doesn't show it's running, but it really is in the background. So, I guess I can't manually install it to fully work with AMP either, bummer.
I assume AMP is using the steamapp cmdline with the Login "anonymous" to download Factorio? Maybe that doesn't work anymore? When I tired anonymous via the cmdline I got the same "No subscription" error. I tried to see if there was another SteamID for a public dedicated server, but I couldn't find one.
Any thoughts on this?