Enabling PlantUML does not work

Hi all,

when I try to enable PlantUML I get a 500. The log says this:

==> /var/log/gitlab/gitlab-rails/production.log <==
Started PATCH "/admin/application_settings" for 127.0.0.1 at 2019-02-26 11:08:08 +0000
Processing by Admin::ApplicationSettingsController#update as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"[FILTERED]", "application_setting"=>{"plantuml_enabled"=>"1", "plantuml_url"=>"http://plantuml:8080"}}
Completed 500 Internal Server Error in 31ms (ActiveRecord: 11.1ms)

OpenSSL::Cipher::CipherError ():

lib/gitlab/crypto_helper.rb:27:in `aes256_gcm_decrypt'
app/models/concerns/token_authenticatable_strategies/encrypted.rb:55:in `get_token'
app/models/concerns/token_authenticatable_strategies/base.rb:27:in `ensure_token'
app/models/concerns/token_authenticatable_strategies/encrypted.rb:42:in `ensure_token'
app/models/concerns/token_authenticatable.rb:38:in `block in add_authentication_token_field'
app/services/application_settings/update_service.rb:18:in `execute'
app/controllers/admin/application_settings_controller.rb:40:in `update'
lib/gitlab/i18n.rb:55:in `with_locale'
lib/gitlab/i18n.rb:61:in `with_user_locale'
app/controllers/application_controller.rb:417:in `set_locale'
lib/gitlab/middleware/multipart.rb:103:in `call'
lib/gitlab/request_profiler/middleware.rb:16:in `call'
lib/gitlab/middleware/go.rb:20:in `call'
lib/gitlab/etag_caching/middleware.rb:13:in `call'
lib/gitlab/middleware/correlation_id.rb:16:in `block in call'
lib/gitlab/correlation_id.rb:15:in `use_id'
lib/gitlab/middleware/correlation_id.rb:15:in `call'
lib/gitlab/middleware/read_only/controller.rb:40:in `call'
lib/gitlab/middleware/read_only.rb:18:in `call'
lib/gitlab/middleware/basic_health_check.rb:25:in `call'
lib/gitlab/request_context.rb:20:in `call'
lib/gitlab/metrics/requests_rack_middleware.rb:29:in `call'
lib/gitlab/middleware/release_env.rb:13:in `call'

Does anyone has an idea whats the problem?

brase