If "cache" means .newsrc.eld, 10s of MB seems rather large. I wonder what the corruption was, since it doesn't sound like anything I can remember in ages using Gnus. .newsrc.eld does have backups in the same way as other files.
If you're missing XOAUTH2 authentication, there is an Emacs auth-source backend for it.
Yes, it was the newsrc.eld which got corrupted. I can't remember the circumstances, but I think it was an Emacs or Gnus bug at that time. This would be over 10 years ago though.
If you're missing XOAUTH2 authentication, there is an Emacs auth-source backend for it.