Anyone who attempts to join my server is greeted with a "failed to download mods" error. I later found out that the packages folder must contain an rfcmp file in addition to a rfmod file. The rfcmp file allows clients to automatically download the correct mods when joining the server, so I am told. The problem is I don't know where these are or how to make them. I searched everywhere on my PC and found none.
What kind of Content do you use? If you use Workshop Content, you can find the rfcmp in the Subfolders of the Folder ..\Steam\steamapps\workshop\content\365960 If you use Content that you've downloaded somewhere and installed it manually, you should have their rfcmp Files in your Packages Folder already.
The host placed all the rfcmp and accompanying files in Packages as shown: https://i.imgur.com/FLle6FZ.png. When attempting to join, the client received this error: https://i.imgur.com/2Owa2tj.png. The client copied all rfcmp and accompanying files to packages, just like the host, and the same error occurred. We tried again and there was no error. Instead, the client's game ran at an incredibly low FPS and used all CPU resources while sitting at the "Click To Start" screen. We thought that the client's game was trying to download something so we let it run for 15 minutes or so. Eventually, we gave up because nothing happened. This happened during all subsequent attempts to join the server.
The client and the host have installed the same content (all the rfcmps), which is all official DLC, no community content. I don't know how "installing" an rfmod file works. The client copied the rfmod that is used by the server into rFactor 2/Packages.