It does not matter what I do.
Disable antivirus
disable firewall
make exceptions
reinstall

Every single game that goes through the "scanning installed files - updating files - verifying files" sequence gets to the 96% complete mark and abruptly goes to "installation failed" status and aborts.

Anyone know why?

Try using the normal offline files from your account, that don't rely on the galaxy client.
https://www.gog.com/account

Try talking to customer service. When they all fail at the same point they should be able to tell you what is happening at 96% to help narrow down the issue.

However, I second FrodoBaggins idea. Get the offline back-up. One place to get them is through your account page. Another place is the extra button next to the install/play button for each game page in galaxy. The offline backup will be second from the bottom on the list. With galaxy you can queue files up and let it do its thing while to do something else.

Just a wild hunch, do you have enough disc space? Those games you are updating have some gigabytes.

I once got a Dying Light update that wanted to allocate 30+GB. Although I do not know how Galaxy handles the allocation of disc space for updates. I am using the offline installers.

As FrodoBaggins said, offline installers solve most problems.

i wonder how hard to make a client that works

Where are you trying to install the games to? Never use Windows special folders (like "Program Files"), Windows has a tight grip on those and the "fail at 96%" usually happens when Galaxy finished downloading everything, but fails to move the stuff into the specified folder - and most often lacking permissions are the problem.

Try installing a game into something like C:\GOGGames and see if that works.

Yep, as some of the others have said, it could a be space related or permission related or another possibility is a drive issue (i.e. bad blocks).

If you have a second drive in your PC, whether physical or a partition, it is usually better to install there rather than an OS drive ... especially if you are running your OS from a limited (smallish) size SSD.

I know that it has been a few months, but this has just started happening to me. Files are scanned, download happens, progress bar goes to 96%, I get the message "executing setup" and then "update failed".

This is not a disk space issue (not for for me anyway). The two updates I am trying to get are small (20MB and 27MB) and the disk has 1.9TB free!

The folder is the folder that I have always used for my GoG games, and it is not a special windows folder.

I have checked the disk several times and there are no errors reported for it.

I don't want to get the offline installers because that means downloading several GB to get a 20MB update.

Until recently, this did not happen and updates always worked just fine.

The updates just keep trying over and over, failing every time.

There has been a Windws update and a GoG galaxy update since the last time that I successfully updated a game. so I guess either of those might be responsible.

Any ideas?


[edit]
The suggested fix on the GoG support page of renaming the dependencies_temp folder to dependencies and merging it with the dependencies folder in the GoG galaxy folder worked.

However, it only worked to a certain extent. I had to rename it for each update. It wasn't a one-shot fixes all thing.

Galaxy issues strike again.

Hmmm... Just harboring a guess, i'll assume you aren't running Galaxy as Admin? Setup failing may be something stupid like the temp folder it can't get permissions to make a folder for it's work, or maybe it's trying to run some other dependencies. Admin may get around that, but i'm always iffy about running games and tools as admin unless you have to.

That's a guess. Another could be to write a script that you can execute to handle said folders so it works, though how often it would have to be executed is a guess.

Control doesn't have an offline backup

Just checked and Control do have a backup installer