Could it be Out of file descriptors! · Issue #100 · haskell/tar · GitHub / cabal haddock failure: out of FDs due to tar · Issue #11131 · haskell/cabal · GitHub? ?
hackage-server still uses tar-0.6: hackage-server/hackage-server.cabal at 350fd8aac61c76481fcccfeee499ab6675171320 · haskell/hackage-server · GitHub