package redmine 3.2.1-2 failed to install: json-1.8.3 not found

However, json-1.8.3 seems to be installed, see output of following command:

$ gem list --local | grep json
json (2.0.1, 1.8.3)
multi_json (1.11.2)

The bundle command recommended along with the error message doesn't work:

$ bundle install
Could not locate Gemfile or .bundle/ directory

ProblemType: Package
DistroRelease: Ubuntu 16.04
Package: redmine 3.2.1-2
ProcVersionSignature: Ubuntu 4.4.0-28.47-generic 4.4.13
Uname: Linux 4.4.0-28-generic i686
ApportVersion: 2.20.1-0ubuntu2.1
AptOrdering:
 tightvncserver: Purge
 redmine: Configure
 NULL: ConfigurePending
Architecture: i386
Date: Thu Jul 7 18:23:54 2016
ErrorMessage: subprocess installed post-installation script returned error exit status 7
PackageArchitecture: all
RelatedPackageVersions:
 dpkg 1.18.4ubuntu1.1
 apt 1.2.12~ubuntu16.04.1
SourcePackage: redmine
Title: package redmine 3.2.1-2 failed to install/upgrade: subprocess installed post-installation script returned error exit status 7
UpgradeStatus: Upgraded to xenial on 2016-07-04 (3 days ago)