Recently, I've noticed people complaining about their Jmod addon failing to download and update. I'm also one of those affected people (only for a limited time) and I also noticed some odd issues when updating games/addons via Steam itself when using certain OSes. So I've decided to do some research about this issue and this is, what I've discovered.

The main cause of this issue is the fact, back in June 2025, Valve introduced new compressing algoritm which their servers use to compress data with, before they send them to the clients. Maybe this looks like a irrelevant issue at first, but here's the catch. If you are using Steam client on Windows 7/Windows 8/8.1, then these OSes aren't supported by Steam anymore and thus your client is out of date. The point is, the old client back from January 2024 (latest version still compatible with Windows 7/Windows 8/8.1) is still used to old compressing algoritm and when it receives data compressed by the new algoritm, it has no clue what to do with them. This is also the reason, why your client ends up with "corrupted data" error on downloading window, when it tries to download/update something and you're on Windows 7/Windows 8/8.1.
If you're using Windows 10/Windows 11, then your client has been already updated, and knows what to do with data compressed by the new compressing algoritm. So, if Jmod fails to download and you're on Windows 10/Windows 11, then your issue lies somewhere else.

If you have a PC/laptop with Windows 10/Windows 11, and you wanna download/update the game on the PC/laptop with older version of Windows, the simplest way is to download the game on Windows 10/Windows 11 PC/laptop and stream it on your LAN network to your PC/laptop with older version of Windows. But be aware this only works with games themselves. This doesn't work with workshop addons!
If you don't have a PC/Laptop with Windows 10/Windows 11 or you want to update workshop addons, then this depends on, what OS are you using.

If you have Windows 7, then you've got lucky, because there's a program called VxKex-NEXT available online on GitHub. Just Google it and don't ask me for a link, I'm afraid the link may get flagged as malicious, if I post it here. Also for instructions, how to use VxKex-NEXT, you have to Google them, since I've never used VxKeX-NEXT myself.

If you have Windows 8/8.1, then this is more complicated. According to VxKeX-NEXT devs, they're trying to make VxKex-NEXT to be compatible with Windows 8/8.1, but it's still unclear when this will be done. In the meantime if you want to download/update the game you need to download the game files using the "Steam depot downloader", copy downloaded files into appropriate folder and modify a "appmanifest" file for your game, so your steam client thinks, it updated the game already.
For downloading "Steam depot downloader" and for instructions how to use Steam depot downloader and how to modify "appmanifest" file, pls Google them. Again, I'm afraid, the link for Steam depot downloader may get flagged as malicious, if I post it here, and instructions how to use Steam depot downloader and how to modify "appmanifest" file will take too long to explain here.
Unfortunately, updating workshop addon manually is very complicated. First you need to download the latest workshop addon file via steam workshop downloader (Google it, they're many workshop downloaders) and replace the old addon file with the one, you've downloaded. The next step is to modify "workshopmanifest" file, so your Steam client thinks, it updated the addon already. But unfortunately this is the part, where I've got stuck and I can't figure it out.

Unlike Windows 7/Windows 8/8.1, Steam clients on Windows 10/Windows 11 have been already updated and can work with the new compressing algoritm. If you have issues with downloads on these versions, verify firewall settings and/or try to clean the download cache. This usually works. If it didn't work, try to reinstall Steam client.

I shouldn't forget to mention, the advice above for Windows7/Windows8/8.1 is meant to be a short term solution. The long term solution for this problem is simple. Upgrade your Windows to newer, still supported version. And if your PC/laptop isn't suitable for newer Windows version due to hardware, then you have two options. Either find a different OS compatible with your PC/laptop and Steam, or wave your PC/laptop goodbye and get a new one.
Also I'm aware, Windows 10 "died" recently this month (14th of October 2025). So if you are using Windows 10, then rest assured, you can still use Steam on it. But be aware you may face the same issue like Windows 7/Windows 8/8.1 users do now in the future, so don't hesitate with upgrading your Windows version.

Also, if someone know, how to modify "workshopmanifest" file, so your Steam client thinks, it updated the addon already, please let me know in discussion below.