I jumped back into Skyrim after about 4 months of not playing it at all. I started a new game, and I suddenly cannot auto attack. With or without mods installed, I just cannot auto attack and I cannot figure out why. I can use power attacks, or use my M2 attack while unarmed.

If I dual wield two weapons, I cannot do a normal attack with either.

Is there a known fix for this?

Disable all mods and delete everything in your character animation folder (Skyrim > Data > Meshes > Actors > Character).