Fixing "This server is running a different inventory version."

This means the Steam item details loaded on your client don't match the server. A good first troubleshooting step is to verify game files which repairs any corrupted or missing files, and checks for updates:

1. Right-click Unturned in your Steam Library
2. Click Properties...
3. Click Installed Files in the left column
4. Click Verify integrity of game files

Unless you manually modified the EconInfo.json file the most likely cause is the file got corrupted. Either way, verifying game files should replace it. Ordinarily this file should only be modified by an official update to the game, but in that case a different error message will be shown saying the version number is different.