After starting up a new world, I started exploring near the spawn point until a sudden crash.
This issue seems to involve several mods including: Embers, Inventory Pets, AbyssalCraft, Recurrent Complex, Astral Sorcery, denseores, and Roguelike Dungeons.

[15:09:10] [Server thread/WARN]: Recurrent Complex loaded a new chunk (-23, 23 Dimension: 0) during chunk population, causing cascading worldgen lag. Please report this to the mod's issue tracker. This log can be disabled in the Forge config.

I tried posting the log in zip, but it wouldn't upload.

capnkirok of Inventory-Pets seems to have the solution:

capnkirok/Inventory-Pets#170

Duplicate of #155.
Their fix will not work for ReC. The main issue is big structures rather than their shift - I have started work on fixing the issue but it will take a little while longer.