I just tried to run:

  darcs changes <file>

in an old repo that I didn't realize was
        Format: darcs-1.0

The resulting message was unhelpful (with a long path elided below):

  Creating a patch index, please wait. To stop press Ctrl-C
  darcs: [elided]/_darcs/hashed_inventory: openBinaryFile: does not exist (No such file or directory)
 
For what it's worth, it works to do just "darcs changes".
 
This is with version "2.9.5 (+ 10 patches)", but there don't appear to
have been any relevant changes since.

Hmm. I thought that I'd fixed this for Issue2237.

Can you check if you built your darcs with the patch

    "resolve issue 2237 prevent patch index creation for non-hashed repos"

If so, and you're still getting the error, then we need to do some more
investigation!

Urk. I see the same behaviour as you, using the oldfashioned.tgz repo in
tests/data/, and runnning `darcs cha x`.

Maybe I didn't catch all codepaths that create the PI.

The following patch sent by Owen Stephens <darcs@owenstephens.co.uk> updated issue issue2253 with
status=resolved;resolvedin=2.10.0 HEAD

* Resolve issue2253 - attempting to use the patch index shouldn't create it on OF repos 
Ignore-this: b1a9ea6c7bf9dd2571258c9d005511ce