NameError: uninitialized constant ActionDispatch::XmlParamsParser trying to install redmine_agile - ruby-on-rails

Question

I'm trying to install the Redmine Agile plugin on a server running Ubuntu 16.04.4. I have a setup with mod_passenger and Apache similar to what's described in this Stack Overflow answer. I seem to be playing error whack-a-mole and have asked a question previously on this. I'm executing these comamnds as www-data.
Here's my console output.
www-data#host:/usr/share/redmine/public/plugins/redmine_agile$ bundle exec rake redmine:plugins NAME=redmine_agile RAILS_ENV=production
`/home/ubuntu` is not writable.
Bundler will use `/tmp/bundler/home/ubuntu' as your home directory temporarily.
(in /usr/share/redmine)
rake aborted!
NameError: uninitialized constant ActionDispatch::XmlParamsParser
/usr/share/redmine/config/application.rb:55:in `<class:Application>'
/usr/share/redmine/config/application.rb:8:in `<module:RedmineApp>'
/usr/share/redmine/config/application.rb:7:in `<top (required)>'
/usr/share/redmine/Rakefile:4:in `require'
/usr/share/redmine/Rakefile:4:in `<top (required)>'
/var/lib/gems/2.3.0/gems/rake-12.3.2/exe/rake:27:in `<top (required)>'
(See full trace by running task with --trace)
If I try to run the command from /usr/share/redmine per this answer, I get a different error.
www-data#host:/usr/share/redmine/public$ bundle exec rake redmine:plugins NAME=redmine_agile RAILS_ENV=production
`/home/ubuntu` is not writable.
Bundler will use `/tmp/bundler/home/ubuntu' as your home directory temporarily.
(in /usr/share/redmine)
rake aborted!
Gem::LoadError: Specified 'mysql2' for database adapter, but the gem is not loaded. Add `gem 'mysql2'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord).
/usr/share/redmine/lib/plugins/acts_as_activity_provider/init.rb:2:in `block in <top (required)>'
/usr/share/redmine/config/initializers/00-core_plugins.rb:12:in `eval'
/usr/share/redmine/config/initializers/00-core_plugins.rb:12:in `block in <top (required)>'
/usr/share/redmine/config/initializers/00-core_plugins.rb:2:in `each'
/usr/share/redmine/config/initializers/00-core_plugins.rb:2:in `<top (required)>'
/usr/share/redmine/config/environment.rb:14:in `<top (required)>'
/usr/share/redmine/lib/tasks/redmine.rake:52:in `block (2 levels) in <top (required)>'
/var/lib/gems/2.3.0/gems/rake-12.3.2/exe/rake:27:in `<top (required)>'
Caused by:
Gem::LoadError: mysql2 is not part of the bundle. Add it to your Gemfile.
/var/lib/gems/2.3.0/gems/bundler-1.16.2/lib/bundler/rubygems_integration.rb:408:in `block (2 levels) in replace_gem'
/usr/share/redmine/lib/plugins/acts_as_activity_provider/init.rb:2:in `block in <top (required)>'
/usr/share/redmine/config/initializers/00-core_plugins.rb:12:in `eval'
/usr/share/redmine/config/initializers/00-core_plugins.rb:12:in `block in <top (required)>'
/usr/share/redmine/config/initializers/00-core_plugins.rb:2:in `each'
/usr/share/redmine/config/initializers/00-core_plugins.rb:2:in `<top (required)>'
/usr/share/redmine/config/environment.rb:14:in `<top (required)>'
/usr/share/redmine/lib/tasks/redmine.rake:52:in `block (2 levels) in <top (required)>'
/var/lib/gems/2.3.0/gems/rake-12.3.2/exe/rake:27:in `<top (required)>'
Tasks: TOP => redmine:plugins:migrate => environment
(See full trace by running task with --trace)
In both directories, gem list mysql gives the same output.
www-data#host:/usr/share/redmine/public/plugins/redmine_agile$ gem list mysql
*** LOCAL GEMS ***
mysql2 (0.5.2, 0.4.3)
www-data#host:/usr/share/redmine/public/plugins/redmine_agile$ cd ../..
www-data#host:/usr/share/redmine/public$ gem list mysql
*** LOCAL GEMS ***
mysql2 (0.5.2, 0.4.3)
Here's the Gemfile in the plugin directory.
www-data#host:/usr/share/redmine/public/plugins/redmine_agile$ cat Gemfile
gem "redmine_crm"
gem "mysql2"
How can I resolve this exception and get the agile plugin working? Thanks!