Update2: Okay! I just discovered that this project has been archived and that yahoo-group-archiver is being recommended (which seems to be doing the trick) https://github.com/IgnoredAmbience/yahoo-group-archiver
Update: Turns out all was working. The 404s were on account of the first few hundred messages being deleted by hand (it was a very contentious group in the early days ;-). Now I'm locked out with a 500, though I am still able to read from my browser. Will let some time pass and try again. Thanks!
I got this far. I set the cookies in archive_grou... (read more)