That is very strange indeed...I have never seen such an issue before. Could you please check that content is being installed to the correct directory, that is the "Installed" directory in the root of your rFactor 2 installation folder, and not elsewhere. Can you also make sure that the "Packages" folder to which the '.rfmod' is being downloaded, when you join the server, is the one in the root of your rFactor 2 installation folder, and not one that is elsewhere; for this you will, of course, need to either rename or delete the existing '.rfmod' file before attempting to join the server.
Others have been able to access the server successfully, which would suggest it is a client-side issue, but it is strange that you are able to access the other RD rFactor 2 server without any problems.
For good measure I have restarted the server, just in case it got itself into a bit of a state.