Which pandemic of 541 AD weakened Byzantium?

The plague reached Egypt in 541 and Constantinople the following year, at the worst possible moment for Justinian, who was in the middle of reconquering the western Mediterranean. Procopius, who was in the city, describes the machinery of a state trying to cope: officials assigned to count the dead, then giving up on counting, then the towers of the city walls being filled with bodies because there was nowhere else. Justinian caught it himself and survived.

For a long time nobody could be sure what it was. That was settled by ancient DNA: teeth from sixth-century graves in Bavaria yielded Yersinia pestis, the same species behind the Black Death, though from a distinct lineage that later died out. So the same organism produced two of the worst mortality events in recorded history, eight hundred years apart, and left no descendants from the first attempt.

It came back roughly every decade or so for two centuries, which is the part that does the damage - a single catastrophe can be absorbed, a recurring one reshapes how many people there are to tax and conscript. It also arrived alongside the volcanic dust veil of 536 and the coldest decades in millennia, which makes disentangling any one cause from the others close to impossible.