@Enrico Parisi's observation is the most likely reason for the failure. You can obtain the credentials (assuming you've set them up in Interoperability | Configure | Credentials) with
 

Set tCreds = ##class(Ens.Config.Credentials).%OpenId(..Adapter.Credentials,,.tSC)
Return:$$$ISERR(tSC) tSC

The tCreds.Username and tCreds.Password properties are available on success.

The Connect() method returns a %Boolean, not an %Status. You should check the ReturnCode and ReturnMessage properties to determine why it's failing.

This is what DTLs are for. The example below shows a scenario using HL7 2.5.1's ORU_R01 message structure, where each OBX segment may be followed by one or more NTE segments. The OBX Set ID, Value Type and Observation Value fields are populated; the first by the value of tObxCounter, the second by the string "TX" and the third by the contents of the Note field. The highlighted area demonstrates the updating of newly added OBX segments with the contents of corresponding NTE segments.

OnGetConnections() should return an array in the first (Output) argument, indexed by the names of the target process(es) and/or operation(s). The second argument is the production item object passed by the invocation of the method from the web ui.

Here's an example that scans the business process settings for any setting that ends with "ConfigName" or "ConfigNames" and sets the pArray argument appropriately:

ClassMethod OnGetConnections(Output pArray As %String, pItem As Ens.Config.Item)
{
	#dim tSetting As Ens.Config.Setting
	Do ##super(.pArray,pItem)
   	For l=1:1:pItem.Settings.Count()
	{
		Set tSetting = pItem.Settings.GetAt(l)
		If ($LOCATE(tSetting.Name,"ConfigNames?$") && pItem.GetModifiedSetting(tSetting.Name,.tValue))
   		{
			For i=1:1:$L(tValue,",") 
			{
				Set tOne=$ZStrip($P(tValue,",",i),"<>W")
				Continue:""=tOne
				Set pArray(tOne)=""
			}
		}
	}
}

As mentioned by @Alexander Pettitt, xfs is recommended, but ext4 is supported as well.

I've used both for LVM2 LVs on RedHat at multiple high-volume customers without issue. For my personal development environments I use Ubuntu 22.04 with LVM2 and ext4 LVs.

It's defined in Ensemble.inc ... but I'm not sure that's what you're looking for. The output should show in the Event log.

#define sysTRACE(%arg)          $$$catTRACE("system",%arg)
#define catTRACE(%cat,%arg)     Do:$$$DoTrace ##class(Ens.Util.Trace).WriteTrace(%cat,$$$CurrentClass,$$$CurrentMethod,%arg)

IRIS For Health includes ASTM E1394-97 / LIS02-A2 structures for mapping to many formats using a DTL, including HL7v2.5.1. It also includes a service class for handling ASTM files (EnsLib.EDI.ASTM.FileService).

Can you share the error message generated by the BPL when it terminates?

You should be able to assign the return value of $EXTRACT(request.{OBX:5},1,510)}* to a context variable and use it in place of the original OBX:5 path in the lookup function. However, if the OBX:5 is longer than the maximum string size in IRIS (i.e. it's a stream), you may  need to include logic to check for that, then read just the first 510 bytes.

* The path to the OBX segment is likely different than what's in my example ... you'll need to supply the correct path.

That setting allows you to specify an adapter address over which arbitration traffic will flow. It's a local address for situations where the arbiter server may have multiple adapters on different networks.

The IRIS servers establish the arbiter connection rather than the other way around, and you can have multiple mirror pairs us a single arbiter instance.

The ISCAgent is normally installed by default with IRIS, but may not be activated. On my mirrored servers, it's active, but I don't remember whether that happened automatically as part of the setup process, or I did it manually.

No allow list that I'm aware of, but if you want to restrict access to ISCAgent ports, add an adapter to each host, connected to a VLAN that is not routeable from outside that network. Configure application_server.interface_address to use it. You could also use the same network for mirror journal transmission/communication, and leverage QoS to allocate a fixed minimum amount of bandwidth that would be unavailable to other network traffic.