Not sure where my other reply went (to the question about 'windows to windows'). The answer to that one was 'Yes, this is between Windows 11 host and Windows 11 VM'.
My clipboard already had HTML unchecked in SlickEdit.
I just did another check and right now the clipboard is working, so it comes and goes. I haven't found a pattern yet, but it is only SlickEdit clipboard that stops working when it happens. I can copy SlickEdit to Visual Code and then Visual Code to VirtualBox when it happens.