Yes, it was because the SSH agent wasn't running (on Windows). I would have preferred to have it error and not submitted the diff, rather than having to do "arc diff" a second time to get the diff pushed to the staging repository.

It can also happen when you configure stagging with ssh and you let the repository beeing clone with ssh and https !

New dev, especially on windows, prefer the https version so it is less an overhead to configure all the stuff. Telling them tha, they MUST configure and use ssh when arc diffing isn't that easy.

But for senior dev, using https isn't an option since ssh makes really life easier (no password to type except maybe your ssh agent once a day) !