I came across the same issue with Add-Change for MXASSET [adding and updating ASSETSPEC records].
BMXAA4129E – The record for Site=TEST, Asset=E9003945 already exists. This validation error is thrown when an attempt is made to insert a new record with a unique key value that already exists in the database. Ensure that the key value for the given record is unique.
The same data set works fine in Mxloader 6.2.
Apologies for negative feedback – i very much appreciate all the effort you have put into Mxloader!