Invisible terrain/chunk issues on a modded server

Hello, I made a custom modpack not too long ago. It works perfect on singleplayer, but upon joining my new multiplayer server, the terrain appears as invisible. All I can see are mobs and lava. When a block is broken (for example, an oak log) the item is dropped, but the block itself still appears as if it were there. It can be passed through by players.

Placing a block updates the chunk, causing all visible terrain to appear in a small radius. It also updates any blocks the player has mined, properly removing their mirage from the world.

I should also note that other players who join have the same exact issue.

This is the most obscure bug I've come across. I've searched online but only found one person with a similar problem, and it was on their singleplayer world. Their easy fix was turning mipmap levels to 0... but that's a client-side graphical option, and this error only happens on this server. I'm lost here.

If anyone knows of a possible fix, I'd appreciate it. And if any pros would like to look through my server files, here's a link to them: https://www.dropbox.com/home/Unbound%20Ventures%20Server

I guess you could say my problem looks a bit like this... only with chunks phasing in and out of existence depending on which blocks the player places and destroys: https://www.youtube.com/watch?v=bi1psFCjLdo

Edit: I reset my server and removed all mods, then logged in with a vanilla client. Everything worked fine. But then I logged on the same server with my custom modpack, and the terrain turns invisible again. Again, this does not happen on any singleplayer worlds with my custom modpack. I am 200% confused.