UPDATE: For some reason, everything seemed to fix itself the next morning (Neverwinter Nights 2 installed with no problems, although the download was at a snail's pace, and Pillars of Eternity 2 is currently downloading and installing), so it must have been something server side for GoG, since nothing changed on my end.

Just letting people know in case they see the post.

Perhaps it has something to do with the free Amazon games, but who knows, I'm just glad everything went smoothly this morning.

Take care and thanks a lot to everyone for the replies and feedback.

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I'm just wondering if anyone has encountered a similar issue or has any idea what might be the cause.

For instance, certain games download and install just fine for me, yet others come up with an error while downloading (E9 - installation failed, after being stuck at 0.0 mbps download speed).

One of the games that has this error is Pillars of Eternity 2: Deadfire. I followed a few different troubleshooting suggestions, i.e. restarting GoG Galaxy, restarting my computer (via hard shutdown and restart), freeing up hdd space, removing 'read-only' from the download folder, downloading all the other free dlc's, refreshing my library, running GoG Galaxy as an administrator, and uninstalling/reinstalling GoG Galaxy, but still no difference.

Further, when I uninstalled GoG and reinstalled it, it didn't recognize that I had Pillars of Eternity 1 installed, and so I tried installing it again and got the same error. Afterwards, I used the GoG Galaxy game search feature and it recognized the installation and I have no problems playing it, but I think that if I uninstall the game now, I won't be able to download and install it again.

Strangely, another game that got this error was Pathfinder: Kingmaker, despite the fact that I had just uninstalled it (so I had previously downloaded and installed it about 3 weeks ago with no problems, uninstalled it about an hour or so ago to make some space (I currently have 350 gb of free hdd space after uninstalling some other games), but can't re-download/re-install the game).

The games I've had no issues downloading and installing are Vampire the Masquerade: Redemption, Might and Magic 6 and 7, and Septerra Core (laggy mess while playing, which has been an ongoing problem, but the download and installation went just fine). So clearly some of the games are downloading with no issues at all (I'm sure many others in my library would download and install with no issues), meaning that it's specific games for some reason.

I also just noticed that I have the same issue when trying to download and install Neverwinter Nights 2 (had no issues in the past until now), and Pathfinder: Wrath of the Righteous (first time trying to install).

Anyways, I'm starting to think this might be a server-side issue (maybe one or more download locations/servers are currently down on their end) or perhaps due to a patch/update (either with GoG or Windows 10), as opposed to something on my end, but I just wanted to get some potential feedback, and see if anyone has had similar issues, either with the same games or even different games.

Thanks and take care

Yes, there appears to be some fault that is out of your control. Myself and another user encountered the server/file not found error when trying to download a specific file (the most current version of) Songs of Conquest backup. But that was yesterday, and this is today; now it downloads correctly. Try POE2 again now, it is working just fine for me. Either way, just try again later.

Same problem, can´t download Warhammenr Rogue Trader. Error E9.

"GOG server returned unknown error" when trying to install Mechwarrior 5 DLC today.

If its server side. The errors are probably due to the large and constant barrage from Amazon freebies. Normal use case all year, does not have so many freebies hammering the bejeezus out of them.

Casual guess, but staff were not about to pay extra for more servers for free stuff....then went....oh crap....we can cook food in the server room lol
(probably not in house, but still a humerous mental image)

Maybe not really related to your issue, but: when I downloaded the updated files for "Neverwinter Nights: Enhanced Edition" yesterday, I encountered a "crawl-speed" (low kb values) while downloading the .bin file.
Well, once started, I thought I let it run.
What else to do anyway, and maybe the pace would pick up, right?

After some time, I logged out of GOG and some time later I also closed my browser (I had totally forgotten about the download at that time).

Now - here's the thing: the download must have had stopped at some point prior to that, because otherwise, my browser would have warned me ("if you close your browser now, your download will be lost" or whatever).
But it didn't.

It was an hour or two later, when I remembered the download and checked my files.
Sure enough, the .bin file was not downloaded completely.

So, I logged into GOG again, and tried to redownload that file - and by then it downloaded just like usual, at normal speed.

Here on GOG, I generally notice some delays from time to time, that I never encounter on other sites (well, not at the same time at least - I sometimes check).

So, yes - there seem to be some issues with GOG's servers.

Same for me. Games are not updating.

All the best

phil