In the "old days," the junkbuster proxy used to return a 128x32(?) blank gif in lieu of an actual block because the page layout would :fu: if the ad wasn't in place and correctly sized. I could easily imagine that might help your situation, too

Don't misunderstand me: it's 100% atrocious that any device bootloops if some ad network 403s, but on the spectrum of "spit into one hand..." and nginx in the other ...