Cohttp installation fails on ubuntu18.04

i am setting up my environment for RWO. When trying to install cohttp on ubuntu18.04 with ocaml 4.06.1:
opam install cohttp -y
gives me:
Sorry, no solution found: there seems to be a problem with your request.
No solution found, exiting

The absence of explanation is quite suspicious. Have you installed an external solver (assuming opam 1.2.x)? If it is indeed a solver problem, simply installing the aspcud Ubuntu package would resolve it.


Thank you very much.