I've run into the same problem, although I'm using cowbuilder-dist instead of pbuilder-dist on Ubuntu 16.04. I've attached a log.
Possibly worth noting - both 18.04 (bionic) and 14.04 (trusty) build correctly - it's only 16.04 (xenial) that fails.
However, both "cowbuilder-dist create trusty" and "cowbuilder-dist create bionic" also generate a similar "Distribution data outdated" warning as the first line of output (as Alexis also mentioned).
I've run into the same problem, although I'm using cowbuilder-dist instead of pbuilder-dist on Ubuntu 16.04. I've attached a log.
Possibly worth noting - both 18.04 (bionic) and 14.04 (trusty) build correctly - it's only 16.04 (xenial) that fails.
However, both "cowbuilder-dist create trusty" and "cowbuilder-dist create bionic" also generate a similar "Distribution data outdated" warning as the first line of output (as Alexis also mentioned).