Organizations that do not have My Domain enabled in their Salesforce instance may receive the above error message when trying to access Taskfeed.
To Configure a Remote Site for Taskfeed:
If working in Lightning Experience, switch to Salesforce Classic
Go to the Taskfeed tab
When in Taskfeed, copy the portion of the URL on your browser as highlighted in the image below:
Go to the Salesforce Setup menu, and in the Security Controls section select Remote Site Settings.
Click New Remote Site.
For the Remote Site details:
Enter Taskfeed for the Remote Site Name
Paste the URL copied in Step 3 for the Remote Site URL
Leave Disable Protocol Security unchecked
Optionally, enter a Description
Make sure Active is checked
7. Click Save. Taskfeed should load as expected now.