Your message is encoded, the actual message text is the following (stored on fnd_new_messages):
Validation failed for the field - &ATTRIBUTE
And ATTRIBUTE appears to refer to the Order Type.
From the payload, the ID for the Order Type is being provided:
Confirm that the value does exist on OE_TRANSACTION_TYPES_ALL table, and notice the org_id.
Other think to notice is the following:
I don't think there is a seeded app with the given short name, double check that the value exists in the fnd_application table. It could be custom, which should be OK but still setup to use MOAC might be missing.
If you are passing this value ('S' )to the MO_GLOBAL.INIT call, try setting it to 'ONT' (the seeded Order Management application) and see if this solves the error.
_ALL tables are multi-org, allowing to define data which is then filtered via the ORG_ID when retrieved with a multiorg view (without the all).
The FND_GLOBAL.APPS_INITIALIZE and MO_GLOBAL.INIT calls will help on setting the org_id as if creating a session on the application, that's why I made a reference to note the ORG_ID when querying the table as the error indicates it is not being found when querying the filtered view, which is read by the APIs.
About using the app_short_name, besides the payload, did you also modify it on the MO_GLOBAL call?