"Error: Please set the API URL" when buying a Free Membership
UPDATE Aug19/2015: The fix is included in 4.6.7

This bug as reported at https://issues.civicrm.org/jira/browse/CRM-14536 seems to be fixed and in QA - if folks can test and report finding that would be great.

The fix is available at: https://github.com/civicrm/civicrm-core/pull/6441

Patch: https://patch-diff.githubusercontent.com/raw/civicrm/civicrm-core/pull/6441.patch 

How to do a patch?