The server pack doesn't let the client join.

https://cdn.discordapp.com/attachments/868783093039919114/1080540434029359114/image.png

[18:38:21] [Netty Client IO #9/ERROR]: Channels [environmental:login,carryon:carryonpackets,mantle:network,storagedrawers:main_channel,environmental:play,antiqueatlas:main,inspirations:network,endergetic:net,upgrade_aquatic:net,quark:main,mannequins:play,paraglider:master,supplementaries:network,mirabilis:mirabilis,pollen:login,patchouli:main,pollen:play,infernalexp:main,geckolib3:main,waystones:network,shulker_enchantments:main] rejected their server side version number
[18:38:21] [Netty Client IO #9/ERROR]: Terminating connection with server, mismatched mod list

no further information given anywhere

I found the solution to this issue that worked, at least for me. The server side is missing two mods, Mannequins and Pollen. Simply copy over your client side versions of those mods over to the servers mod folder and it should hopefully work. Hopefully Vazkii sees this as it is quite a simple fix.