This kick is specific to Unturned. The most common cause is from accidentally installing server plugins into the regular client. BattlEye prevents playing multiplayer with plugins/modules because it would be an easy to way to load cheats. Emptying your Modules folder should fix the problem:
- Right-click Unturned in your Steam Library
- Hover the Manage button
- Click Browse local files in the dropdown
- Open the Modules folder
- Delete all files/folders inside
Why does the game even load modules if BattlEye will kick for them?
The client does actually skip loading modules if it detects BattlEye is enabled. Unfortunately the kick is necessary as an extra precaution, otherwise cheats would force the game to load modules regardless.