Sorry, I haven’t been following the story until now.
Is it “once in a century” simply due to the congested nature of the geographic area beneath the cloud?
That’s just my phrase but yes, the eruption could not have occurred in a worse spot. Not only has it shut down International flights to and from Europe because those routes are right in the path of the cloud (although I think some of the southern routes across the Atlantic are still open) but it has shut down pretty much all air traffic in Europe. It’s had an almost 9-11 types impact on European and International (US to Europe and return) flying.