Beware that $System.SQL.Schema.SetDefault() sets the default schema (optionally) for a namespace systemwide, that is, for all processes, not only current process.

CSPGateway not insllatted in inetbup

How do you check that?

If you select the option you have highlighted, then CSP Gateway will not be installed, if you don't change it, then it will be installed.
What option do you use when you install? (first dialog)

However, is this a new fresh install or an upgrade of an existing Ensemble instance?

Only Client application is Bit 3 only, so in binary:

1000 = 8 decimal

Pass 8 as Type parameter.

On the other hand, are you sure that your method is that same that WRITE uses to interrogate the environment ?

Why should I care? Why do you care?

Maybe your certificate file needs to include the full certificate chain, from your certificate to the CA.

How are you calling the WS?

Are you using a Business Operation with EnsLib.SOAP.OutboundAdapter?

To understand how the header can be customized, can you provide a sample of your SOAP call?

I really, really doubt that the issue is the header namespace prefix.

What does the error (soap fault) says?

EPTEST>Set comp=$system.Encryption.Base64Decode("y61UKEstqlTIyc9LVyguKcoEUuUZmckZCnmpqSnFCiX5CkmpCimpaTmJJakpAA==")
 
EPTEST>Set sc=##class(Community.Compress).UnDeflate(comp,.uncomp)
 
EPTEST>Write uncomp
my very long string which needs to be deflated

Works fine for me

The code I posted does exactly what you need.

You can compress/decompress with deflate or gzip using device I/O.

Then you can download IRIS community edition that you can use for evaluation.

In the main page of the Developer Community click "Download InterSystems IRIS" in the left of the page.

Here is a sample method to deflate without using file I/O and using XDEV instead.

ClassMethod UnDeflate(Compressed As %String, Output UnDeflated As %String) As %Status
{
    Set sc=$$$OK
    Try {
        ; open XDEV device
        Set dev="|XDEV|"_+$JOB
        Open dev:($ZF(-6,$$$XSLTLibrary,12):/HOSTNAME="XSLT") Use dev
        ; write compressed string
        Write Compressed
        ; flush buffers
        Write *-3
        ; change device mode to deflate and rewind
        Use dev:(:/COMPRESS="DEFLATE":/POSITION=0)
        ; read uncompressed content
        Read UnDeflated
    } Catch CatchError {
        #dim CatchError as %Exception.SystemException
        Set sc=CatchError.AsStatus()
    }
    Close dev
    Quit sc
}

I really wish that InterSystems implement the various compress/deflate/gzip functionality available in device I/O as utility methods. Without silly wrappers! 😉

Another wish, please InterSystems document XDEV device I/O.

If I understand correctly you need to deflate and I suppose you want to add the custom wrapper to use $System.Util.Decompress(), if so...you can't. I mean you can't use $System.Util.Decompress() BUT, you can indeed deflate using I/O as in the example I posted in that post/thread.

Beware that OnFailureTimeout() does not work as expected as described in this post.

The issue is present up to 2023.3 and is fixed in 2024.1.