As I am not sure if this issue is due to polybar or herbstluftwm, I thought I would go with my distro instead

So I have rambox installed and launching without issue and the icon appearing in my polybar tray.
As with previous install I had (but not using either polybar or herbstluftwm), when clicking the
icon I get a popout window with options of:

Show/Hide Window
Quit

On my current machine, when I click the Show/Hide Window option nothing appears

Has anyone had this issue? If so, can you point me to what I am looking for to correct it?

Also, please let me know if any further information is required?

So rambox is a "Free and Open Source messaging and emailing app that combines common web applications into one".
The AUR has 2 versions, one of which is apparently out of date - only rambox-bin seems to get enough care, and also upvotes.
So check that.
If that doesn't help I can only recommend testing with different desktop environments/windowmanagers to at least narrow it down.

So I found some worrying output that I am not sure what to do with:
I have also copied my config for both herbstluftwm and polybar to new machine from old (where it works) and that made no difference

I am also running the same versions on both machines

Again, any help appreciated

At what point do you see this output - immediately when you start polybar, or later when you start Rambox? Or only when you click the icon as described in post #1?
Is that the only message you get, or is there some more context around it?

And again, can you reproduce this problematic behaviour with other panels, or is it only with polybar?

Yes to immediately, ie it is one of about 5 lines (the rest just say loading different modules). I can't seem to find any logging for rambox, so not sure what happens when I am cllicking the
"Show/Hide Window" option, but I can say that this does not log anything in polybar's log as above

At present I have not tried any other panels in the new machine. On the old box my standard manager is LXQT and that has never had an issue with it. Also, as mentioned, the old box has both polybar and herbstluftwm as an alernate login and can launch the rambox window when selecting "Show/Hide Window"

Thanks for your interest, if you can think of any place other logging might occur I am happy to track it down? I did look in journalctl but do not see any relelvant info in there

Strange, that seems to suggest that polybar does not load a tray icon for that client at all?
So, if Rambox is already running when you start polybar (is it?) that would suggest that you don't get a Rambox tray icon at all - or that the error message has nothing to do with Rambox.

I found only 2 mentions of your specific error message on the whole internet, and both specify "(XCB_WINDOW (3))", no other number, and have nothing to do with Rambox.

Ok, I think I get what you have said and think I know why.

So both polybar and rambox are loaded from the herbstluftwm autostart file. I think because polybar is loaded prior to other items, such as rambox, which would
give a tray item, on starting it is saying the tray is effectively empty ... which is true until the other items start up.

Sorry for the red herring, but at leat now I know what that message means.

So back to square one on why rambox won't launch / display its window on the new machine

I am wondering if it is that I somehow have something else installed on the old box (which is a lot and some is good and some bad) which is allowing it to display?

Yes, so back to post #2 I guess. Which version did you install? rambox-bin?

After some clicking I found their github repo. Have a good read, the README is long, there's a wiki, and an issue tracker - maybe somebody already asked your question.

Yes to rambox-bin. Will have a look at README

Finally ... so after finding the config file for rambox and comparing to old machine,
the miracle answer is:
Not sure where it grabs this value from as not something I entered. I can only
assume it defaulted based on polybar but my old machine will have defaulted
based on bar in LXQT which used the above

Anyhoo, hope this helps some poor soul who like me was befuddled