What could trigger another euro-zone crisis?

There is currently no sign of a crisis in the euro zone. In particular, yield spreads between the countries are low and stable. But is there reason to fear another euro-zone crisis in the future? What could cause one? Probably, the worst risks are: The current transitory inflation becoming permanent inflation, which would force the ECB to sharply raise interest rates and stop controlling yield spreads; Failure of the “Draghi reforms” and continued zero potential growth in Italy; The introduction of stringent new fiscal rules, forcing euro-zone countries to sharply reduce their fiscal deficits, like after the subprime crisis.