THANKS !   👍

you could try to set your terminator to null String "". 
it is optional accordig to docs
https://docs.intersystems.com/iris20251/csp/docbook/DocBook.UI.Page.cls?KEY=GIOD_tcp#GIOD_tcp_open

• terminators

Optional — A list of up to eight user terminator characters that will terminate reads on the TCP binding device. If you specify both T mode and terminators at the same time, T mode is ignored.

Possible workaround:
 

$ListToString(%DLIST(DISTINCT(REPLACE(ARCIM_Desc,'<br/>',''))),'<br/>') as "ARCIM_Desc", 

	#; Recompute hash here now we have the lock
	Set hashval=$$$hash($lb($$ClassIndexTS^%SYS.cspServer2(db),$roles)_rangelist)

$ROLES of the user might also be involved in the issue

again YES !
Any expansion is related to an extension of the internal mapping structure of the  .DAT  file
written to disk If you don't have any restrictions on disk space, you consume the related
overhead in 1 single shot at a time that you control (e.g., startup)
 

A possible escape from that trap could be to use a dedicated box
That just runs TCPTRACE
So the message runs CLIENT -> BOX -> SERVER -> BOX -> CLIENT

Of course, a qualified network manager might use Wireshark for this issue
 

HMMM.
Are you sure that WS and WSS Protocol are enabled on the Webserver
https://ac1.mqtt.sx3ac.com

And any firewall in between?
 

You are right,  The example defines 
Class WSCI.CRE Extends (%RegisteredObject, %Net.WebSocket.ICredentials)
but doesn't use it,
According to the class definition of %Net.WebSocket.Client you pass it as a parameter
in the %New() method.

 so you code (extending my example may look like this:

 	set cre=##class(WSCI.CRE).%New()
#; feed whatever your credentials need 	
#; init connection 	
 	set ws=##class(%Net.WebSocket.Client).%New(url,cre,evl)

It might be worth checking what class %Net.WebSocket.Client does with your credentials

The article https://community.intersystems.com/post/websocket-client-iris-internal
has a link to OEX https://openexchange.intersystems.com/package/IRIS-internal-WebSocket-Client
which points to code on Github that includes a Client and an Echo Server for testing

the code runs on any IRIS. Docker is just a helping platform

Dmitry, you are just great.
And you impressed me over and over again.
Thank you for sharing your knowledge and your passion 

Point 2. of the previous reply is definitely WRONG!

Point 4. takes that back in some way but leaves it ambiguous.

Using extended Global References (also in Class Storage Definitions)
allows access to any mounted DB if you have access rights.

A personal example:

USER>for i=1:1:5 set ^|"^^C:\InterSystems\IRIS242\mgr\nonspace"|rcc(i)=i
 
USER>zwrite ^|"^^C:\InterSystems\IRIS242\mgr\nonspace"|rcc
^|"^^C:\InterSystems\IRIS242\mgr\nonspace"|rcc(1)=1
^|"^^C:\InterSystems\IRIS242\mgr\nonspace"|rcc(2)=2
^|"^^C:\InterSystems\IRIS242\mgr\nonspace"|rcc(3)=3
^|"^^C:\InterSystems\IRIS242\mgr\nonspace"|rcc(4)=4
^|"^^C:\InterSystems\IRIS242\mgr\nonspace"|rcc(5)=5
USER>

I learned this traditional technique  47 years ago. 
And it still works fine.
 

straight ObjectScript 

USER>if $d(^|"%SYS"|CONFIG("Namespaces"," ")) ;; position $ZR in CONFIG
 
USER>for  set ns=$o(@$ZR) quit:ns=""  zwrite ns  ; list it
ns="%SYS"
ns="SAMPLES"
ns="USER"
 
USER>

BINGO !