Webapp cleanup:tokens rake task fails randomly

We often get these cronmails at night:

rake aborted!
RestClient::Conflict: 409 Conflict
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/rest-client-1.6.9/lib/restclient/abstract_response.rb:48:in `return!'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/rest-client-1.6.9/lib/restclient/request.rb:269:in `process_result'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/rest-client-1.6.9/lib/restclient/request.rb:212:in `block in transmit'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/rest-client-1.6.9/lib/restclient/request.rb:206:in `transmit'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/rest-client-1.6.9/lib/restclient/request.rb:68:in `execute'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/rest-client-1.6.9/lib/restclient/request.rb:35:in `execute'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/couchrest-1.1.3/lib/couchrest/rest_api.rb:94:in `execute'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/couchrest-1.1.3/lib/couchrest/rest_api.rb:60:in `delete'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/couchrest-1.1.3/lib/couchrest/database.rb:200:in `delete_doc'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/couchrest_model-2.0.1/lib/couchrest/model/persistence.rb:60:in `block in destroy'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.21/lib/active_support/callbacks.rb:403:in `_run__4187480936138227802__destroy__877496941063488076__callbacks'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.21/lib/active_support/callbacks.rb:405:in `__run_callback'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.21/lib/active_support/callbacks.rb:385:in `_run_destroy_callbacks'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.21/lib/active_support/callbacks.rb:81:in `run_callbacks'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/couchrest_model-2.0.1/lib/couchrest/model/persistence.rb:59:in `destroy'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/couchrest_session_store-0.3.0/lib/couchrest/model/rotation.rb:24:in `destroy'
/srv/leap/webapp/app/models/token.rb:61:in `destroy_with_rescue'
/srv/leap/webapp/app/models/token.rb:40:in `block in destroy_all_expired'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/couchrest_model-2.0.1/lib/couchrest/model/designs/view.rb:144:in `each'
/srv/leap/webapp/vendor/bundle/ruby/1.9.1/gems/couchrest_model-2.0.1/lib/couchrest/model/designs/view.rb:144:in `each'
/srv/leap/webapp/app/models/token.rb:39:in `destroy_all_expired'
/srv/leap/webapp/lib/tasks/leap_web_core_tasks.rake:21:in `block (2 levels) in '
Tasks: TOP => cleanup:tokens
(See full trace by running task with --trace)

(from redmine: created on 2015-12-02, closed on 2016-03-23)