Hey JJarts, I have a couple problem solving issues that seemed to work for other players. First try the way Mushy has linked, if it continues try the steps below.
1st: Trying verifying your Game Cache. This can be done by following these steps.
Go to your steam library, find Unturned > Right click and go to Properties > Go to the tab Local Files > then click Verify Integrity of Game Cache
A download will start, after it is complete try re-launching Unturned and try joining again. If that does not work try step two.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
2nd: If verifying your game cache does not work try another option, Manually rebooting steam.
Open up your Task Mananger > Look for a program running called "Steam Client Bootstrapper (32 bit) or (64 bit)" > Right click that programs and click End Task > After that relaunch steam, by using your Desktop Icon or Taskbar Icon
It will ask you re-enter your username and password. Re-launch Unturned, If that does not work theres always the last resort which is you may have to re-install Unturned all together.
If after all that, and it still does not work, feel free to find me on TeamSpeak 3 IP: ts.pandahut.net