If you have configured External Sharing successfully for your Org and still cannot see tasks or boards being displayed in the External Sharing link provided, you may be using version 11.22 (Spring 2017) of Taskfeed which has been since updated to 11.23 and onwards.
To install the latest version of Taskfeed:
- Use this URL to install the package into any organization: https://login.salesforce.com/packaging/installPackage.apexp?p0=04t0J000000klPS
Note: If you are installing into a sandbox organization you must replace the initial portion of the URL with http://test.salesforce.com
Please contact Taskfeed Support at firstname.lastname@example.org if you encounter any further issues regarding External Sharing.