The solution above solved the issue.
The content of DSGXB62_Test_PP.xsl I can not share, but it seems to be used for routing to different destinations, instead of processing the content of the file.
Original Message:
Sent: Fri August 04, 2023 06:36 PM
From: Joseph Morgan
Subject: Parse Error and Illegal Character at offset 0
>> I changed it to NULL to match with the K8s instance. Do you think this may be related to the issue?
Certainly, since having INPUT as NULL means the policy won't attempt to parse anything on INPUT. I suspect the DSGXB62_Test_PP.xsl isn't attempting to process anything in the flow.
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Joseph Morgan
Original Message:
Sent: Fri August 04, 2023 05:37 PM
From: Brian Alejandro Villarruel Velazquez
Subject: Parse Error and Illegal Character at offset 0
Hello!
Thank you all for your answers.
About the versions used:
New server, CPC VM with RHEL 8: DP IDG.10.5.0.2
Old server on IBM Cloud Kubernetes: IDG.10.0.1.11
I noticed a difference between the two DP instances in the transformation rule of a processing policy:
The new (failing) had 'INPUT' and 'OUTPUT' on their respective fields, and I changed it to NULL to match with the K8s instance. Do you think this may be related to the issue?

Unfortunately we were not able to get more testing today
I will try getting more logs and enabling the probes and come back with more information. Should I continue on this thread or open a new one?
Thanks!
Original Message:
Sent: Wed August 02, 2023 06:27 PM
From: Brian Alejandro Villarruel Velazquez
Subject: Parse Error and Illegal Character at offset 0
Hello! we are doing a migration and as part of this my team installed DP 10.5.0.2 in a virtual machine.
We are currently facing a problem for AS2 inbound messages, we get a Parse Error.

Checking the logs I can find a little more information where the error indicates Illegal Character 'T' at offset 0.

This character 'T' lines up with the first character on the messsage content. So it seems it is reading the content of the message and encountering an error with it.
It is strange because on the DP instance we are migrating from (in Kubernetes), the same message is succesfull, we exported and imported all configuration from this instance, and I have been comparing all the configurations I can think off on both DataPower instances but with no success.
(AS2 Handler, B2B Gateway, Partner Profiles, B2B routing file, etc...
Has anyone encountered this issue before or have an idea of what could be causing it?
Thanks!