opam 1.2.0 is broken in practical terms. It is no longer compatible
with the current archive format.

The developers are recommending migrating to opam 1.2.2.

As far as I know, Debian and Devuan Jessie are the only distros still
shipping opam 1.2.0.

Would it be possible to provide 1.2.2 in Jessie, possibly in Jessie
backports?

I suppose opam-docs should be upgraded in the same way.

-- hendrik