Redmine

The workaround is to comment a line in the mailer.rb

  def reminder(user, issues, days)
  #  set_language_if_valid user.language

Environment:
  Redmine version                3.2.6.stable
  Ruby version                   2.2.6-p396 (2016-11-15) [i386-mingw32]
  Rails version                  4.2.7.1
  Environment                    production
  Database adapter               Mysql2
SCM:
  Subversion                     1.8.5
  Mercurial                      2.9.2
  Git                            1.9.4
  Filesystem
  GitRemote                      1.9.4
Redmine plugins:
  clipboard_image_paste          1.10
  redmine_agile                  1.4.3
  redmine_checklists             3.1.5
  redmine_close_button           0.0.8
  redmine_git_remote             0.0.1
  redmine_issue_clipboard        0.1.0
  redmine_last_updated_by_column 0.3.0
  redmine_lightbox2              0.2.7
  redmine_people                 1.3.0
  redmine_questions              0.0.7
  redmine_slack                  0.2