Forge version: 36.2.39
The Hordes mod version: The-Hordes-1.16.5-1.1.5

No hordes spawns. Some log errors when loading a world.
latest.log

What does your config file look like, that's a parsing error for an incorrect config file.

Changed file extension to load a file.
hordes-common.txt

Only changed spawnFirstDay to true

What other mods do you have installed?

Tested with and without Improved Mobs.

Hordes actually spawns, but not the first when config option spawnFirstDay is set to true.