This was unfortunately a mistake made by upstream Debian. Rather than reimplement the constraint solver badly, we use the Aspcud external solver. An upgrade of Aspcud changed its command-line interface, and OPAM 1.1.0 wasn't upgraded to OPAM 1.1.1 at the same time (which detected the new version of Aspcud and worked with it).
A similar issue (to do with the libdose interface) has been in the Ubuntu package ages and not addressed by upstream despite a detailed bug report: https://bugs.launchpad.net/ubuntu/+source/opam/+bug/1401346
> Searching online found recommendations to get the latest version from Github
What else could we recommend?
It's really frustrating to know that a source-compiled OPAM works fine, but that the packaged binary versions are broken and our only recourse is to wait six months for the next OS release of Ubuntu. On the bright side, the OPAM 1.2.x series has been very stable and is now making its way upstream, so this is hopefully a temporary issue.