I finally managed to install GG Max 17/03/2021 last week, using the new Tiny Installer. It took 23 hours to download, but it finally finished and I was able to run it.

This morning I started it and ran the updater to bring it up to the 26/03/2021 version. That was going OK, but then it crapped out on the third last file and reported errors in install_errors.txt. These lines were listed about 27,000 times in the log file:

HTTP download failed for file "Max/Files/texturebank/terrain grey grid_color (uncompressed).dds", HTTP status: 200, Internal Error: "None"
HTTP download failed for file "Max/GameGuruMAX.exe", HTTP status: 200, Internal Error: "None"

Attempting to verify the file produced the same failure.

So now when I try to run GG Max (regardless of the install errors), I get this from Windows 10:

This app can't run on your PC
To find a version for your PC, check with the software publisher

GG Max now won't run at all, even under admin permissions. This is not the first time this has happened, I've uninstalled and wiped GG Max in the past for exactly the same reasons. I thought it had finally come good with the 17/03/2021 version, but now it's failed again.

Any advice on how to get the missing file, or otherwise sort out the updater/installer?

23 Hours !!
Thats pretty extreme .. I would say 23 minutes is a little slow.
What is your internet speed ?
At that rate im dubious whether your getting all the files downloaded correctly.

https://github.com/TheGameCreators/GameGuruRepo/issues/909

I had to start from scratch with the tiny installer to fix it.

I managed to fix it myself. I successfully installed GG Max onto another machine in my house. I identified the missing files and copied them from that machine to my main one, and after that the GG Max install on my main machine came good.

The missing files were:
-- Max/Files/texturebank/terrain grey grid_color (uncompressed).dds as reported above. It looks like that file just got corrupted and was only partially downloaded. A partial copy in the target folder may have prevented it from downloading properly on subsequent attempts to install the update.
-- Max/GameGuruMAX.exe, i.e. the main program file. Maybe that file downloads last in the list or something and the update process had already crashed before it got to this file. Whatever, it didn't get downloaded, hence Windows 10 told me that the app wouldn't run when I opened the system link to it.

The difference between my two machines is that the main one is further from the router, has a lower signal strength, and for some reason appears to handle downloads of single files poorly (as opposed to downloads of a large archive file). This is weird, but I guess I have to put the download failures I've experienced down to some kind of local network latency problem. Anyway, fixed now. Thanks all ...

I would suggest buying yourself a couple of wifi extenders, one plugs into a plug socket near your router and the other plugs in near your PC, that should help with your signal.