After days of going through a troubleshooting hell, studying the issue, and receiving help from some users, I bring you the definitive step-by-step guide to fix this infamous issue.
 
Your feedback is greatly appreciated!
 
 

Quick heads-up

– This guide is for client-hosted servers only (“HOST” option on the main menu), NOT dedicated servers.
 
 
– The instructions will do for anyone experiencing the problem, but when “[HOST-ONLY]“ is present on a step, it means it should only be followed by the user/s who are trying to create and host a server.
 
 
– If you’re planning on playing with friends, I advice the whole group to follow the steps at the same time, with patience and communication.
 
 
With that clear, let’s start.
 
 
 

PART 1: Base fix

1- Create a collection – [steamcommunity.com]  with all the mods you wanna use (In case you don’t already have one).
 
 
2- Unsubscribe from all the mods in it using “Unsubscribe from all” on the collection page.
 
 
3- a. Delete the \Steam\steamapps\workshop\content\108600 folder.
 
ㅤ b. Delete the Steam\steamapps\workshop\appworkshop_108600.acf file.
 
ㅤ c. [HOST-ONLY] Delete the \Steam\steamapps\common\ProjectZomboid\steamapps\workshop\content\108600 folder.
 
ㅤ d. [HOST-ONLY] Delete the \Steam\steamapps\common\ProjectZomboid\steamapps\workshop\appworkshop_108600.acf file.
 
 
4- Restart Steam.
 
 
5- Subscribe to the collection again (If the download isn’t automatically starting, run Project Zomboid and close it).
 
 
6- [HOST-ONLY] Once Steam finished downloading the workshop content:
 
ㅤa. Paste the \Steam\steamapps\workshop\content\108600 folder into \Steam\steamapps\common\ProjectZomboid\steamapps\workshop\content.
 
ㅤb. Paste and replace the \Steam\steamapps\workshop\appworkshop_108600.acf file into \Steam\steamapps\common\ProjectZomboid\steamapps\workshop.
 
 

At this point you should already be able to play without and issue, but keep reading to know how to preserve your server!

 
 
7- [HOST-ONLY] Create a new server, tweak it however you like WITHOUT ADDING MODS and save a backup by clicking in “Manage settings…” and “Duplicate Selected Settings” (Just to be safe).
 
 
8- [HOST-ONLY] Add the mods to your server via the “Steam Workshop” tab, only then, check which files you want to keep in the “Mods” tab.
 
(This is just in case a mod offers more than one archive and requires you to choose which one you keep. If you don’t have a mod that does this, you can ignore this step).
 
 
 

PART 2: Keeping the server through time – “Up-to-date and/or backup”

Okay, so we have 2 options here. They’re not day and night on the practice, not quite opposite, but different enough to make for a dichotomy, even though opting for one the first time doesn’t mean you won’t be able to perform the other in the future.
 
Remember: If you will be adding/removing mods, check the first advice on the “Tips” section.
 
 

Option A: Up-to-date server

 
Make sure to only follow this option’s instructions IF YOU ARE A HOST.
 
This one is for those who want their mods fresh and updated on their servers. It’s not “THAT safe” since you never know when a mod author’s gonna mess up a little and break their mod beyond user-end solution, (Even though that’s pretty rare) but other than that, there shouldn’t be any other possible problem with this. You can always perform option B instructions at any point in time. Keep in mind that you will have to perform these steps everytime a mod receives an update, (a.k.a you get the workshop error while trying to join your server) even though that’s only gonna happen a few times at most. Luckily, these ones are notoriously simpler than any of the other procedures on this guide.
 
 
1- a. Delete the \Steam\steamapps\common\ProjectZomboid\steamapps\workshop\content\108600 folder.
 
ㅤ b. Delete the \Steam\steamapps\common\ProjectZomboid\steamapps\workshop\appworkshop_108600.acf archive.
 
2- a. Paste \Steam\steamapps\workshop\content\108600 folder into \Steam\steamapps\common\ProjectZomboid\steamapps\workshop\content.
 
ㅤ b. Paste \Steam\steamapps\workshop\appworkshop_108600.acf archive into \Steam\steamapps\common\ProjectZomboid\steamapps\workshop.
 
 

Option B: The backup

 
Once you’ve followed all the steps above, I bet you don’t want your modlist to break the day after. Well, better to prevent than to cure. Your mods won’t be updated.
 
 
1- Copy the \Steam\steamapps\workshop\content\108600 folder and keep it safe and sound inside a pretty nice backup spot on your PC.
 
 
If you’re a host, I highly advice to upload your backup to some site and share it with your group of friends just so you all have a 1:1 mods folder backup and there will be NO way strange stuff will happen afterwards. It’ll make things 1000% easier for everyone.
 
 
The next and final 2 steps are the most important and will require you to perform them everytime your mods get updated (Don’t worry, won’t happen that often) or your server throws the workshop error to anyone trying to connect who previously didn’t have any trouble logging in (Assuming they already followed the steps 1-5 of this guide correctly).
 
 
2- If you’re a HOST:
 
 
ㅤa. Delete both \Steam\steamapps\workshop\content\108600 AND \Steam\steamapps\common\ProjectZomboid\steamapps\workshop\content\108600 folders.
 
ㅤb. Copy and paste your “108600” folder backup into both \Steam\steamapps\workshop\content AND \Steam\steamapps\common\ProjectZomboid\steamapps\workshop\content (Remember, DON’T JUST PASTE AND REPLACE, THAT’LL BREAK THINGS. First delete, then paste).
 
 
ㅤIf you’re a PLAYER:
 
 
Delete the \Steam\steamapps\workshop\content\108600 folder and paste your backup there (The same goes for you. First delete, then paste. Never paste and replace)
 
 
3- [HOST-ONLY] Paste the \Steam\steamapps\workshopapp\workshop_108600.acf archive into \Steam\steamapps\common\ProjectZomboid\steamapps\workshop
 
 
(“appworkshop_108600.acf” is Project Zomboid workshop’s “manifest” archive. That means it validates the status of the mods in your game and will always stay updated just by being stored in \Steam\steamapps\workshop, so that’s why you gotta copy and paste it side by side with your backup everytime WITHOUT saving it INSIDE the backup, since that would prevent it’s automatic update executed by Steam whenever a mod is updated, so there’d be no use for that archive being in your backup cause it’d be outdated. Then, it would cause Steam to start updating your mods right after reading it. What we are trying to achieve with this archive is to make Steam think all your mods are up to date, always, when that’s clearly not the case).
 
 
 

Tips

– “Phased”/”Lost” worlds can be recovered. If you ever went through the somewhat familiar experience of getting locked out of a server’s world cause you couldn’t find a way to fix the workshop error on it, then you gave up and moved on, created another world on another server slot but never actually deleted the previous one, then it happened AGAIN and now you’re here reading this, you know what I’m talking about.
 
First you have to remove all mods exclusively named with NUMBERS from the “Steam Workshop” tab on the server settings (Mods that don’t exist anymore in your client cause you unsubscribed from them and removed it’s archives but still do in the server settings will appear named as their workshop ID instead of their original, real names. You can look up the ID on the workshop). Performing this before/after (Doesn’t matter) following the 1-6 steps guarantees access to the world without losing any progress except from the one related to removed mods content.
 
 
– If you see an error written in red over a black screen after trying to boot your server, then you are facing either a broken mod (Incompatible with the current build/Poorly built) or a mod compatibility problem. Most of the times, the string should give you an idea of which mod is causing the problem. Try performing step 6 from part 1, but it that doesn’t work out, don’t waste time, go ahead and remove it right away.
 
 
– If you entered your server but none of your mods are showing, make sure the contents of your \Steam\steamapps\common\ProjectZomboid\steamapps\workshop\content are in place. There should only be a folder named “108600”, which contains all the mod subfolders inside.
 
 
– If you need any extra help, don’t hesitate to contact me!
 
 
 
Here’s my personal fully functional lore-friendly focused workshop collection for 41.60+, if you’re interested:
 
https://steamcommunity.com/sharedfiles/filedetails/?id=2680940720 – [steamcommunity.com] 
 
 

 
 
This is all for Project Zomboid How to Fix – Workshop item version different than server’s (41.X) Guide hope you enjoy the post. If you believe we forget or we should update the post please let us know via comment, we will try our best to fix how fast is possible! Have a great day!