Registering the OSV blocks causes the ore's loot tables to be loaded immediately, which will fail if the loot table contains any modded items. Changing the loot table in question to drop a vanilla item rather than a modded one allows the loot table to load, so my assumption is that modded items aren't registered yet, since the item in question definitely eventually exists.

Crash log: crash-2020-11-19_19.57.21-fml.txt

What I've tried:

• Setting the RegistryHandler#registerBlocks event priority to lowest, which doesn't seem to alter this behaviour.
• Changing RegistryHandler#registerBlocks to use a DeferredRegister instead - doesn't alter this behaviour, definitely defers registration as the call to BaseOreVariant#updatePostRegister throws with this change.

Presets now get loaded even later than they previously were in 5.11, which should mitigate this issue even more. I'm gonna go ahead and close it.