I recently upgraded to 5.39.0 on WP 5.8. When clicking continue from an event registration form at the Confirm Your Registration Information stage, I consistently get the sent to the url suffix https://example.com/civicrm/event/register/ and have the yellow bar page with the message:
Sorry, due to an error, we are unable to fulfill your request at the moment. You may want to contact your administrator or service provider with more details about what action you were performing when this occurred. Failed to find key by ID or tag (:ImagineAKeyHere:)
The participant is successfully registered when looking in the events. Also at other odd times that I have not discerned a pattern a black error box chimes in with
Failed to find key by ID or tag (:ImagineSameKeyHere:)
The baseURLs for CiviCRM and Wordpress are identical per the respective GUIs.