This is, understandably, an annoying kick. For an explanation of why it's important, please take a look at the Behind the Scenes section at the bottom of this article.
The game automatically checks that you have the same version of any workshop files used by the server, so the most likely reason for the kick is that the file has been corrupted, locally modified, or modified on the server.
If it's only happening for you (and not other players on the server), then the problem is likely with your copy of the file. A good first troubleshooting step is to verify game files, which repairs any corrupted or missing files and checks for updates:
- Right-click Unturned in your Steam Library
- Click Properties...
- Click Installed Files in the left column
- Click Verify integrity of game files
Otherwise, if it's happening for other players on the server as well, the problem is likely with the server's copy of the file. The server host can perform similar steps by following this article: Repairing server workshop install
Behind the Scenes
When Unturned 3.0 was first released (a long time ago now!), there were no asset integrity checks. Over time, some players realized they could modify asset files locally and use those modifications in multiplayer. For example, they were setting the recoil of guns to zero.
To prevent this, we began verifying the gun configuration when equipping an item on the server and canceled the action if it didn't match. This became a game of whack-a-mole - checking everything from vehicles to attachments to weather on a case-by-case basis as cheaters tried to exploit everything.
Admittedly, a broader approach should have been taken sooner, but it was low-priority as the problem developed slowly over ~7 years. Eventually, however, we replaced all of these case-by-case checks with a more effective general solution: the integrity of all assets used by the level and server is verified in multiplayer, as well as the Unity asset bundles they loaded from.
So, although it is frustrating when you get kicked for this, there is hopefully some silver lining to know that it's for a good reason: it raises the barrier to cheating so that cheaters must use external programs that can be detected by the anti-cheat and permanently banned.