Redmine

Hello everyone,
we recently started using redmine and our staff wishes do be able to reply directly to emails to comment on tickets.
i installed redmine on a vm as explained here:

http://www.redmine.org/projects/redmine/wiki/howto_install_redmine_on_ubuntu_step_by_step

the user credentials are OK and tested successfully (making them faulty leads to an approbiate error message)

No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.2 LTS
Release:        18.04
Codename:       bionic

Environment:
  Redmine version                3.4.4.stable
  Ruby version                   2.5.1-p57 (2018-03-29) [x86_64-linux-gnu]
  Rails version                  4.2.10
  Environment                    production
  Database adapter               Mysql2
SCM:
  Subversion                     1.9.7
  Darcs                          2.12.5
  Mercurial                      4.5.3
  Cvs                            1.12.13
  Bazaar                         2.8.0
  Git                            2.17.1
  Filesystem                     
Redmine plugins:
  no plugin installed

ruby -v
  ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux-gnu]

sudo bundle exec rake --trace -f /usr/share/redmine/Rakefile redmine:email:receive_pop3 host=HOST username="DOMAIN\\USER" password="PASS" 
/usr/lib/ruby/vendor_ruby/sprockets/digest_utils.rb:47: warning: constant ::Fixnum is deprecated
/usr/lib/ruby/vendor_ruby/sprockets/digest_utils.rb:51: warning: constant ::Bignum is deprecated
/usr/lib/ruby/vendor_ruby/sprockets/processor_utils.rb:110: warning: constant ::Fixnum is deprecated
/usr/lib/ruby/vendor_ruby/sprockets/processor_utils.rb:111: warning: constant ::Bignum is deprecated
** Invoke redmine:email:receive_pop3 (first_time)
** Invoke environment (first_time)
** Execute environment
/usr/lib/ruby/vendor_ruby/builder/xchar.rb:111: warning: constant ::Fixnum is deprecated
** Execute redmine:email:receive_pop3
rake aborted!
FrozenError: can't modify frozen String
/usr/lib/ruby/2.5.0/net/pop.rb:808:in `block in pop'
/usr/lib/ruby/2.5.0/net/protocol.rb:276:in `each_message_chunk'
/usr/lib/ruby/2.5.0/net/pop.rb:955:in `block in retr'
/usr/lib/ruby/2.5.0/net/pop.rb:1013:in `critical'
/usr/lib/ruby/2.5.0/net/pop.rb:953:in `retr'
/usr/lib/ruby/2.5.0/net/pop.rb:807:in `pop'
/usr/share/redmine/lib/redmine/pop3.rb:49:in `block (2 levels) in check'
/usr/lib/ruby/2.5.0/net/pop.rb:666:in `each'
/usr/lib/ruby/2.5.0/net/pop.rb:666:in `each_mail'
/usr/share/redmine/lib/redmine/pop3.rb:48:in `block in check'
/usr/lib/ruby/2.5.0/net/pop.rb:532:in `start'
/usr/share/redmine/lib/redmine/pop3.rb:43:in `check'
/usr/share/redmine/lib/tasks/email.rake:148:in `block (4 levels) in <top (required)>'
/usr/share/redmine/app/models/mailer.rb:433:in `with_synched_deliveries'
/usr/share/redmine/lib/tasks/email.rake:147:in `block (3 levels) in <top (required)>'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/task.rb:271:in `block in execute'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/task.rb:271:in `each'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/task.rb:271:in `execute'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/task.rb:213:in `block in invoke_with_call_chain'
/usr/lib/ruby/2.5.0/monitor.rb:226:in `mon_synchronize'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/task.rb:193:in `invoke_with_call_chain'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/task.rb:182:in `invoke'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/application.rb:160:in `invoke_task'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/application.rb:116:in `block (2 levels) in top_level'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/application.rb:116:in `each'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/application.rb:116:in `block in top_level'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/application.rb:125:in `run_with_threads'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/application.rb:110:in `top_level'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/application.rb:83:in `block in run'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/application.rb:186:in `standard_exception_handling'
/var/lib/gems/2.5.0/gems/rake-12.3.1/lib/rake/application.rb:80:in `run'
/var/lib/gems/2.5.0/gems/rake-12.3.1/exe/rake:27:in `<top (required)>'
/usr/local/bin/rake:23:in `load'
/usr/local/bin/rake:23:in `<main>'
/usr/local/bin/ruby_executable_hooks2.5:24:in `eval'
/usr/local/bin/ruby_executable_hooks2.5:24:in `<main>'
Tasks: TOP => redmine:email:receive_pop3