SOLVED The entity type x does not have an "revision_created" entity revision metadata key.

Add this to your entities annotations to fix the error (modulename/src/Entity/entityname.php):

*   revision_metadata_keys = {
*     "revision_user" = "revision_user",
*     "revision_created" = "revision_created",
*     "revision_log_message" = "revision_log",
*   },