This is the message I am getting when I attempt to load my game.

Failed to load mods: Error while running setup for recipe prototype "ufp_voidarmor1_recipe" (recipe): Recipe item-product "ufp_voidarmor1" is not stackable but has a max count of 2.

The game wants me to disable the following mods:

This one
Castra Prime
BiggerStackPlus
Rigor-module

The options I can see for BiggerStacksPlus are pretty limited, and I am not seeing an option that looks like it would change the stack for armor.

Any ideas what the conflict could be and what I can do to fix it?

so, im with factorio 2.1, ive tried your list of mods and it worked without crashs here:
UFP
Castra Prime
BiggerStackPlus https://mods.factorio.com/mod/BiggerStacksPlus
Rigor-module
i added UMR that have a item stack editor too enabled, and it dont crashed, its another mod of yours thats editing item stacks

send your factorio-previous.log and i will take a look to help

I appreciate your response. I got it figured out by tinkering around on my own.

Basically, BiggerStacksPlus was attempting to modify the stacks on items the game deemed "unstackable". I had to add exceptions to the LUA file in BiggerStacksPlus.