Please see the list of possible solutions for the frontend user below:

Having a setting or plugin that blocks third party cookies aggressively will cause this error.
Look in your browser setting and try setting it to allow third party cookies.

Cookies that are failing to update or that contain unusable data.
Clearing the cookies can solve this problem. 

Internet security software may interfere and create this error.
Try turning it off momentarily to test the page.

The session may have expired if you left the page open for a long time.
Refresh the page.