I have heard a ton of people mention this for water world (though may be something for others). This is actually an easy fix. This error means that the map you currently have is not the same version as the one on the server. This could mean that: 1.) you downloaded an older version of the map on the workshop separately 2.) the one downloaded from the server is old. To fix this if you match with 1, just simply unsubscribe from it on the workshop. If you did not download it from the workshop and it came directly from Ice-mod, then you need to go to this directory (Windows) : C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\download\maps. Look for the issue map .bsp. Then delete it. The next time you go on IM, the newer version should download when the map is next played. Hope this helped some of you!