diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index f8d0b4f31..a25aa20e6 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -47,7 +47,7 @@ jobs:
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 2.5.5
+          ruby-version: 2.6.5
       - name: Increase MySQL max_allowed_packet to 1GB (workaround for unknown/missing service option)
         run: |
           sudo apt-get install -y mysql-client
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index a55293b50..09f0eae6e 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,4 +1,4 @@
-image: registry.znuny.com/docker/zammad-ruby:2.5.5
+image: registry.znuny.com/docker/zammad-ruby:2.6.5
 
 # Global variables added to the ENV of each job
 variables:
@@ -60,8 +60,11 @@ variables:
       alias: postgresql
 
 # Cache gems in between jobs and pipelines
+# ATTENTION: We use a combination of the Ruby major and minor version number
+# as a key for the cache to avoid cache growth and incompatibilities between
+# the gems. Search hook: 2.6.5 (change it when updating the Ruby version too)
 cache:
-  key: "ruby25"
+  key: "ruby26"
   paths:
     - vendor/ruby
 
diff --git a/.ruby-version b/.ruby-version
index 0cadbc1e3..57cf282eb 100644
--- a/.ruby-version
+++ b/.ruby-version
@@ -1 +1 @@
-2.5.5
+2.6.5
diff --git a/Gemfile b/Gemfile
index 643b46577..422488559 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,7 +1,7 @@
 source 'https://rubygems.org'
 
 # core - base
-ruby '2.5.5'
+ruby '2.6.5'
 gem 'rails', '5.2.4.1'
 
 # core - rails additions
diff --git a/Gemfile.lock b/Gemfile.lock
index af36949ab..229fed331 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -263,7 +263,7 @@ GEM
       addressable
       faraday
       json (>= 1.8)
-    libv8 (6.7.288.46.1)
+    libv8 (7.3.492.27.1)
     listen (3.1.5)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
@@ -287,8 +287,8 @@ GEM
     mimemagic (0.3.3)
     mini_mime (1.0.2)
     mini_portile2 (2.4.0)
-    mini_racer (0.2.4)
-      libv8 (>= 6.3)
+    mini_racer (0.2.9)
+      libv8 (>= 6.9.411)
     minitest (5.13.0)
     msgpack (1.2.4)
     multi_json (1.14.1)
@@ -666,7 +666,7 @@ DEPENDENCIES
   zendesk_api
 
 RUBY VERSION
-   ruby 2.5.5p157
+   ruby 2.6.5p114
 
 BUNDLED WITH
    1.17.3