And after about 3, I get HTTP Error 403 for the rest, can't resume, have to start them again. What gives?
Specifically I'm currently trying to download Horizon Zero Dawn, all 19 parts. I like to use my download manager so I can set them to go one at a time instead of all at once. Anybody know what's going on?

It's been brought up before. Download links expire after a while. Try downloading the offline installers through GOG Galaxy instead of a browser or download manager.

Been a while since I needed to use FDM, but I have read a few times here now, that GOG have set download links to die after something like 10 minutes, which was rather nasty of them and clearly another effort to force customers to just use Galaxy.

However, if you are prepared to try out some command-line GOG SDK based downloaders, like gogcli.exe or gogrepo.py etc, then you can achieve what you want. They let you queue up downloads and only grab the link as required.

NOTE - If you have a flaky connection, then Galaxy may be your only solution when it comes to an easy resume. That said, there is a way to resume with FDM with a link that has died, if you know your way around FDM. Unfortunately there is no simple (fuss free) way to use FDM now unless you can download your files within the limited time frame GOG allow.

Can confirm that this is NOT the case. I'm still on a rather slow (but therefore consistently stable) connection and downloading one 4 GB .bin chunk through the browser/library takes about 40 minutes. And I hadn't one single instance where the download would just stop and get cancelled after 10 minutes.
Hardly ever had any issues initiating the concurrent download of several of the 4 GB .bin chunks, logging out and leave it downloading over night either. If one or two of the downloads ended up incomplete it was due to getting into the time frame when the ISP does its compulsory reconnection every 24 hours.

I've noticed in recent downloads, resuming doesn't work. Maybe the session expired, i don't know, though i'd prefer if it didn't and i didn't have to restart from scratch on said file(s). I used it all the time 2017-2019, along with bulk youtube downloader. I'd get a page of interesting news, or multiple videos, get all links and set to download 30-80 files at a time in quick succession.

Alas, no more.

The link doesn't die while downloading, so no cancel involved. The only issue would be if you lost your connection to GOG and it was after the period of expiry for the link, so no auto resume. This is what some have been experiencing.

As for queued downloads with FDM, have you tried lately?
From the reports I have read it is queued links dying after a time period of about 10 minutes after being added to the queue.

Until a few months ago I was forced to use FDM 5 a lot, but GOG appear to have fixed whatever the server issue was or just paid for more, and so my download speeds with gogcli.exe have been good enough not to need FDM, and so I haven't personally gotten around to checking what has been reported by many now.

Mea culpa, forgot that there's a difference between active and queued downloads for a minute there.
Can't say anything in regards to using any kind of managing solution either, since as described earlier the standard procedure of just downloading several files concurrently through the browser works well enough in my case.

Still doubt that the expiring download links after mere minutes is a thing, though. The "logged in"-token (which is part of every download link) being reset after something like 24 hours of inactivity seems more reasonable, although there were some instances in the recent past, where I had kept the machine running for 3-4 days straight, had not logged out of GoG for those 72-96 hours and had left the library entries (with the download links on display) open for several games in separate browser tabs and still was able to download everything without any indication that the "logged in"-token had been reset at all. I still would have had the compulsory ISP reconnect happening every 24 hours but apparently this wouldn't have affected the "logged in"-token either?

Well aside from other GOG customers having claimed that has happened to them, other sites do exactly that. Itch.io is the perfect example where queued links die after several minutes, even if you are logged in.

So did anyone actually test out this theory about links going bad and preventing broken downloads from being resumed?