Last couple of weeks I have been having issues with trying to load a large modpacks.
I've used the PBO filler mod from a previous pbo issue to test it, but also used workshop mods.
Issue seems when enabling Battleye it block's the ability for a player to join a server.
When disabling Battleye and using the exact same mods players can join fine.
This does happen however when a number of PBO's have been met.