Console also gives no errors and just shows that a player attempted to join and displays their UUID

its exactly as the screen says, you have an outdated mod or maybe additional mod that the server has not installed, please compare your mod folder and versions to the ones on the server

I understand that it's saying there's a mod mismatch. I triple checked. I am running the server build that was released at the same time as the version of the pack (client).

Downloaded it from CurseForge. Is there possibly an incorrect upload on curse?

I experience the same issues with Version 45 and 44. Client and Server had the same Version of the Modpack installed (Server downloaded via curseforge and Client via AT Launcher)
Checking the mods folders I can see that a bunch of mods are outdated on the Server. Too many to just Update them manually by myself.