In many instances, especially when dealing with non-official servers (servers that aren't for OPs), each server may have settings, mod additions, or other things that you do not. So to have such errors appear isn't out of the ordinary.
If you've installed the mods correctly (there's a video in the recruit section showing step-by-step how to install all the mods), then you've nothing to worry about. Again, unofficial servers tend to have additional mods, maps, settings, and so forth, that differ from your own. So don't use them as any kind of standard for mod installation.