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

[Koen G]

I used Google Gemini to clarify the usage of the phrase in Sachs. (I gave it a pdf of the text and the LLM output looks reliable in this case).

In the scene you described from Hans Sachs' Shrovetide play Der Bauer im Fegefeuer (The Peasant in Purgatory), the peasant Heinz Düppel has just been drugged and moved to a dark cell to trick him into believing he has died and gone to Purgatory.

The Context of the Scene
Heinz wakes up in absolute darkness after being given a sleeping potion by the Abbot. He is disoriented and frightened because he cannot see or hear anything and feels only "four stone walls" around him. He tries to call for his wife, Els, to let him out, but a monk disguised as a spirit tells him he is now in the "fire house" (Purgatory) to suffer for the sin of his extreme jealousy.

The "Potz" Phrases
The phrases "Potz Lung" and "potz Leber" (literally "By [God's] lungs" and "by [God's] liver") are euphemistic oaths.

Origin: "Potz" is a corrupted, "softened" version of the German word Gottes (God's).

Purpose: During the 16th century, taking God's name in vain was considered a serious sin or even blasphemy. To avoid this while still expressing strong emotion, people replaced "Gott" with the similar-sounding "Potz" and attached it to various body parts (like lungs or liver).

Usage in the Text: Heinz uses these phrases twice:

• Upon waking up: To express his shock and confusion at being in a "dark hole".
  
• When complaining about his wife: Later, when he learns his wife "offered" a candle at his funeral that burned out before he could use it in Purgatory, he exclaims "Potz Lung' und Leber" to call her "stupid enough" for not providing him with light in the darkness.
  

In the "Peasant in Purgatory" story, Heinz Düppel is the stupid and sinful (jealous) character everyone is supposed to make fun of, the dumb comic relief. The audience mocks his belief that he has really gone to purgatory, which is actually a ruse by the other characters to punish him for his jealousy. It is this character that uses all the potz-phrases.

Interestingly, in this early modern German, the phrases are clearly used as an exclamation: "By God's liver and lung, where am I!?"

The sermon I shared is earlier (1449) and the author talks about the phrases in the context of perjury and false oaths.