Wherever leather strips are in the game, a giant red sign, with an exclamation point inside, shows up instead. I'm currently re-downloading my game to see if it was a vanilla bug or one of my mods was messing with it. Frostfall and Bandoliers - Bags and Pouches are the only mods I have that mess with crafting. Everything else is either a follower mod, player house, item or quest mod. Until I can rule out whether or not it's a vanilla bug, any ideas or suggestions to fix this would be greatly appreciated.

I would say look for a mesh in your data/meshes folder that is for leather straps, if you find one, rename it temporarily to see if that solves the problem.

It in all likelihood is a mod. What you are seeing is the Exclamation Mark of Doom, indicating that a mesh is missing. If you read up on how to use TES5Edit, I imagine you can find the culprit. It sounds like something changed the mesh path for it. Did you remember to install all the asset files for mods?