Hi Soumya,
There are differences between 5.15.1 and 6.10.3 regarding the previous xml to domain mapping and the renamed import mapping. More info you can find here:
https://www.mendix.com/blog/mapping-document-improvements-mendix-6/
Given that you didn't give a lot of information I would, in your case, look to re-configure the service and mapping. You can also set trace in the "Console" ==> "Advanced" ==> "Set log levels" to see what you're sending as request and what you're getting in return(or not).
Regards,