Hi,
this morning I made a small test : I signed a file from the local X
session (I get a window for the passphrase), then I changed keyboard and
tried to sign a file from remote : that worked (nothing asked).
I think that confirms my hunch it's a problem of fallback : if the
session from remote doesn't need a password (because it was just typed
correctly on the X desktop), then things go well. It's only when the
session from remote has to ask for the passphrase that things go awry
and bomb out.
I hope that helps,
Snark on #debian-science