Some info on the bug and resolution:
This affects pipelines created or updated in version 1.5.6, and then upgrading to 1.5.7 version of the plugin. The bug is in how the 'buildcard' setting is set to if there is no value in XML. This 'no build card value' will occur on any pipeline created on any older version, so there is code to set a default value to handle such cases. However, the code does not work for pipelines created or updated in 1.5.6 - it hits a condition where it is not run.
The patch fixes this migration code so that it works with all older versions.
As a work-around you may also re-save the configuration of your pipeline - this also updates your configuration to the correct default value.