In this enwiki Village Pump thread, a user reports getting
Request from 2a02:c7d:c50:c400:882e:902e:b6a:2ba7 via cp3056 cp3056, Varnish XID 987116681 Error: 503, Backend fetch failed at Thu, 12 Nov 2020 07:04:32 GMT.
errors when editing List of shipwrecks in November 1869 and several other similarly large pages with lots of flag icons. Editing individual sections instead of the whole page at once apparently works (but is not a viable workaround for their needs).
Possibly related: T248796