"This server is using a modified version of the map."

This means the map files loaded by your client don't match the map files loaded by the server. The error message should probably be changed because this could also mean the map has been updated/changed, or your local copy might have been modified. (for example if you opened it in the level editor)

A good first troubleshooting step is to verify game files which repairs any corrupted or missing files, and checks for updates to the game and workshop subscriptions:

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
5. Wait for Steam to scan the Unturned install folder and any workshop folders. Steam will then download and install any missing or corrupted files

If this happens with a custom map you made I would recommend setting a version number in your map's Config.json file. The version comparison happens earlier in the connection process, so if you increase the version number before each update players will get a better error message showing the different version numbers.

Advanced troubleshooting

If you are interested in the technical side of things and would like to narrow down exactly which files are mismatched you can add the "-LogLevelHash" command-line option to both the client and server:

1. Right-click Unturned in your Steam Library
2. Select Properties...
3. Find the Launch Options field
4. Type "-LogLevelHash" without quotes
5. Find your dedicated server install
6. Open your server launch script that contains +InternetServer or +LanServer
7. Insert "-LogLevelHash" without quotes before +InternetServer or +LanServer
8. Connect to the server and load into the map
9. Open the log files in a text editor. (For help finding the log files please refer to these articles: Where are the log files? and Where are the dedicated server log files?)
10. Search (usually Ctrl+F) for "level hash"

The log files will contain several lines like "Including [...] in level hash: ####". If the hash does not match between client and server that means XYZ is the mismatched file. If the order does not match that suggests files have been added or removed e.g. a new terrain tile.