XML processing (XPath query) and encoding error

Hi everyone,

I am processing some XML fragment I am getting from a RESTful call using the following operation:
/Base [1.9.1]/Library/Utility Operations/XML Processing/XPath Query

This is working just fine in my Studio, once the content pack with my flow is imported on the central this no longer works and I am receiving the following error message:

(exception=Invalid byte 2 of 3-byte UTF-8 sequence.), (returnCode=-1)

Or from the central's log:

INFO   | jvm 1    | 2017/07/19 10:22:34 | [Fatal Error] :1:1: Content is not allowed in prolog.
INFO   | jvm 1    | 2017/07/19 10:22:48 | [Fatal Error] :3:49: Invalid byte 2 of 3-byte UTF-8 sequence.

How can I solve this problem? The original fragment has no XML prolog (<?xml version="1.0" encoding="UTF-8" standalone="yes"?> for example) and I tried to add the XML prolog and it didn't solved the problem.

Any ideas? Someone encountered this problem before?