Everything about "pox leber" as a minced oath, and an earlier source.

[Koen G]

I looked for some Hans Folz Fastnachtspiele, and they are indeed earlier than Sachs (late 15th century compared to mid 16th). He uses "pox" in the same way:

• Interjections, exclamations -> expressing emotion rather than meaning
  
• spoken by farmers
  

I feel like a broken record here, but context matters.

He also wrote twelve Fastnachtsspiele, short plays that made light of people in medieval society, for instance farmers, priests, and the bourgeoisie.

Words spoken by farmers and similar folk, explicitly intended to make fun of them and the way they speak.