The payload of the key contains only a variable length record dataset. The record consists of an 8 byte unsigned, an 8 byte float, and a UNICODE string. the number of records is also variable.
I note this because we have keys in Alpharetta that have multiple fixed length datasets that have max counts. These keys were in use under 5.4 and 5.6 versions.
I did modify the key to make the child dataset in the payload have fixed length records and a max count, but the failure still occurred.