When adding a custom pathfinder's quill location via the config, underscores in the biome id are not parsed properly. Presumably, the same issue applies to namespace id. I suspect this is a simple issue where the parser assumes the id only contains [a-z:].

crash-2025-02-22_01.41.31-fml.txt

Update:

It turns out that the syntax is consistent with other config options: You must provide each option within quotes, separated by commas. For instance,
Customs = ["minecraft:warm_ocean,2,8,14,27A6B2", "minecraft:deep_dark,5,48,64,18395A"]

Hopefully, this helps anyone experiencing problems with this.