Hi,
New release of monetdb is not installing on ubuntu 16.04 LTS. I think problem will be your new release which is happened on 1 May 2017.
Change https://dev.monetdb.org/downloads/deb/dists/xenial/monetdb/binary-amd64/Release:

Valid-Until: Tue, 30 Nov 2038 00:00:00 JST   this is not working. JST to some other proper format