I can't be sure but it might have been sulfur dioxide
gas. This has a very sharp and irritating odor and is the
odor acid fixing baths produce. Kodak wash aid is mostly
sodium sulfite buffered with some sodium bisulifte. With age
sodium sulfite oxidizes to the sulfate. Sulfate does not
smell and does no harm but is much less effective than
sulfite as a wash aid. What I think might have happened is
that the bisulfite may have decomposed releasing some
sulfuric acid which in turn reacted with the sulfite to
release the gas. I am not a chemist so I am not sure of this
reaction. There may have been some elemental sulfur produced
which would take the form of a cream or yellowish residue in
the container.