Minecraft Version: 1.15.2
Packed Up version: packedup-1.0.12-mc1.15.jar
Tiny Progressions version: tinyprogressions-1.15.2-4.0.1.jar

Steps to reproduce:

1. Create Modpack with Packed Up (Backpacks) and Tiny Progressions
2. Load Game
3. Try to load into a world
4. Game Crashes with Error Message:
   The game crashed whilst ticking memory connection
   Error: java.lang.RuntimeException: Why did the advancements not load yet?!

I am also submitting this report to Kaskdeya (Tiny Progressions developer)

Crash Report:
https://pastebin.com/NMPLG5di

Issue seems to be resolved by Tiny Progressions

I'll see what the Tiny Progressions dev can make of this over on their issue, because I really have no clue why this is happening.