Only do cache cleanup if cache already exists.

config/application.rb

@@ -62,7 +62,6 @@ module Zammad
     # Version of your assets, change this if you want to expire all your assets
     config.assets.version = '1.0'
 
-    
     # Use a different cache store in production
     config.cache_store = :file_store, 'tmp/cache/file_store'
   end

config/initializers/cache.rb

@@ -1,5 +1,7 @@
 # clear cache
-Rails.cache.clear
+if Zammad::Application.config.cache_store[1] && File.directory?(Zammad::Application.config.cache_store[1])
+  Rails.cache.clear
+end
 
 # to get rails caching working, load models
 Dir.foreach("#{Rails.root}/app/models") do |model_name|