Give the gift of Drupal. All merchandise is 50% off through 2016.
This issue is mainly meant to save those some debugging time who run into the same error message.
When changing the field_ns_prod_news_topic to single select there is a problem with the data type of the list argument passed in the "Clear article cache" rule. It leads to the following error:
Fatal error: Cannot use object of type EntityDrupalWrapper as array in /.../profiles/nodestream/modules/contrib/rules/includes/rules.plugins.inc on line 597