Hello, I recommend you contact SnapLogic Support if you have not done so already as they can help you with this situation.
That said, during pipeline validation we only process 50 records so sometimes errors in larger datasets don’t surface until execution. In this case, the XML Parser is telling us that in row 1, column 507, there is an unexpected character. If you can open the XML file in a program that shows row and column number you should be able to navigate to the spot and find the unexpected character. You could also use an online free XML validator like https://www.w3schools.com/xml/xml_validator.asp.