Ever since controllable switched to using framework, I have seen a few reports of the server rejecting the players when they install framework on their clients, since framework is a library for all your mods, then it is also server sided.

So if the server is modded, it will refuse to let the player join citing that they need to remove the framework mod, but the player is able to join fine if the server is vanilla from a few reports I have read.

btw I have no forge knowledge so my justification might be off