Redmine

Hi I am really new to redmine and ruby on rails. I am struggeling with my redmine on Ubuntu 20.04

My Environment:

Redmine version 4.0.5.stable
Ruby version 2.7.0-p0 (2019-12-25) [x86_64-linux-gnu]
Rails version 5.2.3
Environment production
Database adapter Mysql2
Mailer queue ActiveJob::QueueAdapters::AsyncAdapter
Mailer delivery smtp
SCM:
Subversion 1.13.0
Cvs 1.12.13
Bazaar 3.0.2
Git 2.25.1
Filesystem
Redmine plugins:
easy_gantt 1.12
redmine_checklists 3.1.18
redmine_default_assign 0.6
redmine_image_clipboard_paste 3.3.0
redmine_questions 1.0.2
redmine_tags 4.0.0
redmine_theme_changer 0.4.0
wiki_graphviz_plugin 0.8.0

I`ve ran into following error:

We're sorry, but something went wrong.
We've been notified about this issue and we'll take a look at it shortly.

I followed a tutorial an added "gem 'psych'" to my Gemfile.local. That fixed that error but now I am unable to look into redmine tasks, throws Internal error 500.

Apache error.log says:

Completed 500 Internal Server Error in 326ms (ActiveRecord: 138.4ms)

ActionView::Template::Error (Tried to load unspecified class: ActiveSupport::HashWithIndifferentAccess):
    1: <% if defined?(sidebar_tags) && !sidebar_tags.empty? && !@issue %>
    2:   <%= stylesheet_link_tag 'jquery.tagit.css', plugin: 'redmine_tags' %>
    3:   <%= stylesheet_link_tag 'redmine_tags', plugin: 'redmine_tags' %>
    4:   <h3><%= l :tags %></h3>