Redmine is down after attempt to setup smtp configuration - ruby

Question

Today I set up a Redmine instance on Ubuntu Server 20.04 for the first time. That worked out quite well so far. So afterwards I set up a Mattermost instance and installed the Redmine Messenger plugin to receive status updates from Redmine in Mattermost. That also worked brilliantly.
Then I wanted to activate email notifications. That unfortunately did not work.
I did the following:
fabrice#redmine:/etc/redmine/default$ sudo nano configuration.yml
and added the following content (with my information)
# Outgoing email settings
production:
email_delivery:
delivery_method: :smtp
smtp_settings:
address: smtp.example.net
port: 25
domain: example.net
authentication: :login
user_name: redmine#example.net
password: redmine
Then I restarted apache and then redmine was down. See image
errormessage
So I deleted the configuration.yml file and restarted apache. Unfortunately that didn't help. I couldn't take any usable information from the logs. (Which doesn't mean there aren't any)
If anyone has an idea how to get Redmine working again, I would be very grateful.
Last lines of redmine production log:
[ActiveJob] Enqueued ActionMailer::DeliveryJob (Job ID: 93923d8a-4e24-4467-847c-b2bdd774fd07) to Async(mailers) with arguments: "Mailer", "settings_updated", "deliver_now", #<GlobalID:0x00007fe514b9c070 #uri=#<URI::GID gid://redmine-app/User/1>>, #<GlobalID:0x00007fe514b938a8 #uri=#<URI::GID gid://redmine-app/User/1>>, ["login_required", "password_min_length"], {:remote_ip=>"192.168.178.174"}
Redirected to http://192.168.178.88/redmine/settings?tab=authentication
Completed 302 Found in 101ms (ActiveRecord: 62.6ms)
[ActiveJob] [ActionMailer::DeliveryJob] [93923d8a-4e24-4467-847c-b2bdd774fd07] Performing ActionMailer::DeliveryJob (Job ID: 93923d8a-4e24-4467-847c-b2bdd774fd07) from Async(mailers) with arguments: "Mailer", "settings_updated", "deliver_now", #<GlobalID:0x00007fe514b674d8 #uri=#<URI::GID gid://redmine-app/User/1>>, #<GlobalID:0x00007fe514b66bf0 #uri=#<URI::GID gid://redmine-app/User/1>>, ["login_required", "password_min_length"], {:remote_ip=>"192.168.178.174"}
[ActiveJob] [ActionMailer::DeliveryJob] [93923d8a-4e24-4467-847c-b2bdd774fd07] Rendering mailer/settings_updated.text.erb within layouts/mailer
[ActiveJob] [ActionMailer::DeliveryJob] [93923d8a-4e24-4467-847c-b2bdd774fd07] Rendered mailer/settings_updated.text.erb within layouts/mailer (3.9ms)
[ActiveJob] [ActionMailer::DeliveryJob] [93923d8a-4e24-4467-847c-b2bdd774fd07] Rendering mailer/settings_updated.html.erb within layouts/mailer
[ActiveJob] [ActionMailer::DeliveryJob] [93923d8a-4e24-4467-847c-b2bdd774fd07] Rendered mailer/settings_updated.html.erb within layouts/mailer (1.1ms)
[ActiveJob] [ActionMailer::DeliveryJob] [93923d8a-4e24-4467-847c-b2bdd774fd07] Performed ActionMailer::DeliveryJob (Job ID: 93923d8a-4e24-4467-847c-b2bdd774fd07) from Async(mailers) in 28.37ms
Started GET "/redmine/settings?tab=authentication" for 192.168.178.174 at 2021-04-13 11:44:09 +0000
Processing by SettingsController#index as HTML
Parameters: {"tab"=>"authentication"}
Settings cache cleared.
Current user: admin (id=1)
Rendering settings/edit.html.erb within layouts/admin
Rendered settings/_general.html.erb (12.0ms)
Rendered settings/_display.html.erb (15.5ms)
Rendered settings/_authentication.html.erb (13.0ms)
Rendered settings/_api.html.erb (4.0ms)
Rendered settings/_projects.html.erb (9.1ms)
Rendered settings/_users.html.erb (12.4ms)
Rendered queries/_columns.html.erb (6.2ms)
Rendered settings/_issues.html.erb (22.5ms)
Rendered queries/_columns.html.erb (3.3ms)
Rendered settings/_timelog.html.erb (8.1ms)
Rendered settings/_attachments.html.erb (5.7ms)
Rendered settings/_notifications.html.erb (0.4ms)
Rendered settings/_mail_handler.html.erb (8.4ms)
Rendered settings/_repositories.html.erb (12.9ms)
Rendered common/_tabs.html.erb (129.0ms)
Rendered settings/edit.html.erb within layouts/admin (129.3ms)
Rendered admin/_menu.html.erb (2.2ms)
Rendering layouts/base.html.erb
Rendered layouts/base.html.erb (7.4ms)
Completed 200 OK in 147ms (Views: 109.9ms | ActiveRecord: 32.0ms)
Edit:
sudo cat error.log | grep redmine
[ 2021-04-15 08:41:59.5086 21857/7f6e840bb700 age/Cor/App/Implementation.cpp:304 ]: Could not spawn process for application /usr/share/redmine: An error occurred while starting up the preloader.
/usr/share/redmine/config/application.rb:3:in `require'
/usr/share/redmine/config/application.rb:3:in `<top (required)>'
/usr/share/redmine/config/environment.rb:2:in `require'
/usr/share/redmine/config/environment.rb:2:in `<top (required)>'
[ 2021-04-15 09:31:14.7130 34932/7f564044f700 age/Cor/App/Implementation.cpp:304 ]: Could not spawn process for application /usr/share/redmine: An error occurred while starting up the preloader.
/usr/share/redmine/config/application.rb:3:in `require'
/usr/share/redmine/config/application.rb:3:in `<top (required)>'
/usr/share/redmine/config/environment.rb:2:in `require'
/usr/share/redmine/config/environment.rb:2:in `<top (required)>'
[ 2021-04-15 09:36:11.5553 38233/7f07082a9700 age/Cor/App/Implementation.cpp:304 ]: Could not spawn process for application /usr/share/redmine: An error occurred while starting up the preloader.
/usr/share/redmine/config/application.rb:3:in `require'
/usr/share/redmine/config/application.rb:3:in `<top (required)>'
/usr/share/redmine/config/environment.rb:2:in `require'
/usr/share/redmine/config/environment.rb:2:in `<top (required)>'
[ 2021-04-15 09:44:59.2308 38883/7f837c1b4700 age/Cor/App/Implementation.cpp:304 ]: Could not spawn process for application /usr/share/redmine: An error occurred while starting up the preloader.
/usr/share/redmine/config/application.rb:3:in `require'
/usr/share/redmine/config/application.rb:3:in `<top (required)>'
/usr/share/redmine/config/environment.rb:2:in `require'
/usr/share/redmine/config/environment.rb:2:in `<top (required)>'