SAP Integration Suite Splitter will get this error when there is a subsequent API call be for the associated gather for the splitter.

Reason: The gather expects XML but the result of the API call is JSON, hence the error.

Solution 1: use and JSON -> XML converter after the API call

Solution 2: modify your gather to use a different aggregation strategy in the Gather module as follows:

Incoming Format: Plain Text

Aggregation Algorithm: Concatenate