This article is to explain the eBay error "Errors: Validation of the authentication token in API request failed."
This error is caused by a hiccup in the system, and is relatively harmless. All that needs to be one is to sync out the listing to clear the error. To do this, you'll go into the edit listing page for the affected listing, by clicking on the little cog by the listing:
Then simply clicking the Save Changes button in the upper right:
Once this is done, you should see the listing enter a pending full sync status, and shortly thereafter, an In Sync status.
If this solution does not clear the error, or if you have a large amount of listings with this error, please reach out to support!