First, let me preface this by saying I understand that the server file downloads are not a top priority to the Kinetic Staff, however, I still feel it's necessary to bring up. Not too long ago I attempted to create a private server using the Infinity pack through aternos.org which lists it as one of the available options from Technic. However, it came to my attention that the server files includes a mod called Woot, which is not is the current version of the client, and thus I cannot join it. Adding the mod to my client manually of course caused an illegal client error. Seeing as I cannot access the server mod files myself in this instance I'm rather stuck.
Not wanting to be 'that guy,' but from my view the fix is pretty painless, remove the mod from the server files so it matches, and Aternos will update it's resources on it's own when it regularly does. If there's more to it than that, then I apologize for my ignorant point of view. I just want to be able to play with my friend, and not deal with item restrictions of a public server.