This has been confirmed to be a problem with an OPAM binary compiled with OCaml 4.05.0, due to changed semantics in the execvpe call.
Workaround: Run
eval `opam config env`
in your shell before executing the opam command.
There is also a proposed PR in OCaml to regain the old execvpe behaviour in future releases, and @altgr is investigating backporting a fix into the opam 1.2 branch.