@timofthefaintinggoats I have the same problem, started a week or so ago, I think with the latest update. I'm on 6.9.3405.3 (Official Build) (64-bit)
I was using --disable-features=PrintCompositorLPAC
on load, but I removed that and the problem persists.
Print works in chrome and elsewhere on my desktop, but I cannot do anything with it on Vivaldi other than cancel as I think you noted. Very frustrating. Otherwise Vivaldi is almost flawless. I have used it for years.
....UPDATE! I was using "--disable-features=PrintCompositorLPAC" and had the problem. Took it out and still had the printin problem. Now I have put it back in and the printing problem has been resolved! Weird, but at least it works now.