[!] There was an error parsing `Gemfile`:

This error occurs from version 2018 1.2 (platform 04.00 and higher)

Error during upgrade installation containg some of the following.

[!] There was an error parsing `Gemfile`:

[!] There was an error parsing `Gemfile`: cannot load such file --

#  plugin 'rys-bundler', github: 'easysoftware/rys-bundler', branch: 'master'
 >  Plugin.hook('rys-gemfile', self)

Bundler cannot continue.

It is caused by outdated version of redmine installer.

Solution:

Before installing the upgrade

$ gem install redmine-installer