Problem Building Schema: Cannot get Connection from Datasource?

While adding a line to an Assign activity there seems to be an issue with the a part of a variable. Upon expansion the part does nothing, and this means that likely there is an error with loading a schema. Unfortunately the issue is not blatantly apparent; in order to get more detail double-click on one of the existing lines and expand the variable in question until the "Exception - Problem Building Schema" message is visible.

To get more information, right-click on the error message and click "Toggle Display Details"

The following error is one of many possibilities that may be seen in the error box.

Exception: Exception occurred while getting connection: oracle.ucp.UniversalConnectionPoolException: Cannot get Connection from Datasource: java.sql.SQLException: Listener refused the connection with the following error:
ORA-12526, TNS:listener: all appropriate instances are in restricted mode
Cannot get Connection from Datasource: java.sql.SQLException: Listener refused the connection with the following error:
ORA-12526, TNS:listener: all appropriate instances are in restricted mode

The issue in this case was around the MDS, it had recently undergone maintenance and had not been properly released to the development team yet. However, if the server is expected to be readily available refer to the link below on fixing the issue.


Resources
http://oraclequirks.blogspot.com/2008/03/ora-12526-tnslistener-all-appropriate.html