Has there ever been an issue with the freezing of air inside a cryogenic/LH2 vent system?

Yes, numerous incidents have occurred where frozen air (which contains oxygen) has built up within a hydrogen process or vent system. These incidents with vent systems incorporate more than just a vent stack, but include a vent system consisting of additional atmospheric equipment (such as a tank) where the equipment stays cold and allows air into the system in contact with a cold hydrogen stream. 

Vent systems are at risk since they are “open” to the atmosphere and certain flow conditions might result in the aspiration of air into a cold hydrogen flow which then leads to the freezing of the air. 

A small quantity of solid air can create an explosive hazard which then leads to cascading failures from the initial incident. Solid air in hydrogen is also shock sensitive which can lead to unexpected ignition.