Yep, oxygen relief valve issue was on the Centaur second stage. This valve is critical in regulating LOX flow as pressure changes during ascent.

This is a dual engine Centaur, specifically for Starliner, so perhaps it's not as well tested as the more standard Centaur.

Might not help that so much of ULA's Centaur expertise is focused on the next generation Centaur (V) for Vulcan.

What rocket are they using?

Edit: After looking at the story I see that it is a ULA Atlas V.

Also, a quote "The next available launch window for the mission is Tuesday night, but no decision was immediately made for when a second liftoff attempt would be made."