I am trying to download (offline installers) the quickstart guide for the game Legends of Murder Collection, and I get a 404 error.
Any suggestions?

I am not contacting support first because the last 2 times I needed support it took weeks to get a non bot reply, and by that I mean I never got a reply by a non bot, and the problems were solved otherwise.

In most GOG games the manuals, PDF files and such are included in the main game directory. So, if you can't get them from the 'Extras' section you should try looking for them there.

I had (and still have) the same problem since release day.
https://www.gog.com/forum/general/check_these_2_classic_games_from_ziggurat_interactive_a0221/post14
I mean, it's only a quickstart guide, but it sucks if things don't work the way they should.

First thing I checked, not the case with this game. This doesn't fill me with confidence. Did you contact GOG support?

Might try clearing cache, and restarting the browser and/or computer.

Would take gog 2 months to reply to a support ticket for this and they wouldn't even understand it, it would be an obvious rush job reply.
Have had similar issues, wait forever, and they just don't comprehend English well enough to understand or help.
Pretty sure the last time I tried to get them to help me with an issue (because they can't list things properly in their store, causing me to buy something I already owned), they just couldn't understand. And shocker, now that entire game is gone from my catalog, and waiting another 2 months for them to reply, which of course, they won't understand.

I just don't get it, they have like 10 customers and can't keep up with support. They want to be the next steam, sell out their values by adding more and more DRM, but they can't even fully staff customer support?

Error 404 means that the link you clicked on is pointing towards something that isn't there. Although it can be a browser caching issue if you have downloaded it previously and the file name has since changed such that it is trying to download the file with the old name, it is usually something that has to be fixed by however maintains the website. You can try refreshing the page then clearing your browser's cache/cookies or even a different browser but if none of that works, you'll need to contact GOG about it.

Having worked in customer/tech support in the past, the odds are extremely low that if you contact GOG support that you'll get someone who knows what website error codes actually mean. It's not their job to fix any part of the website when it isn't working as it is well above their pay grade, and they don't have the access to do it either.

You'll need to explain the issue to the without expecting them to fix the website. I would say something like:

"Hello. I am getting an Error 404 when attempting to download the quickstart guide for the game Legends of Murder Collection, which means that the link to this file is broken either due to the file being missing or the URL pointing to it being incorrect. Would you be able to email this file directly to me at (your_email_address_here) or send me a direct link to this file so that I may download it?"

More information on Error 404:
https://www.allconnect.com/blog/404-page-not-found-error-explained
https://en.wikipedia.org/wiki/HTTP_404

If you have GOG Galaxy installed, you can go to the game in your library, click on extras and try downloading it from there. If this doesn't work, let me know - I'll see if I can find the quickstart floating about the Internet.

Nice of you. Thank you very much. :)

Here on GoG i get 404 sometimes, clearing cache and restarting the browser seems to fix it.

Never had issues downloading the installers though.

@ponczo_ I'm having issues trying to download the original, plain Quake offline installer.

I keep getting a 404 error. Even after clearing cache and using a different computer nothing seems to work.

Would you, or anyone on the GOG team be able to assist?

You're not the only one having 404 issues....the problem is pretty
widespread(essentially GOG's data hosting is having problems at the moment).

I am also having Error 404 when I try to download the second part of Batman Arkham City outside of Galaxy. And to make matters worse, it's the only way I can download games from my library, as my downloads on GOG Galaxy always stop at 92%
Att: resolved