When using the HTML5 client, the clipboard is not functional even when the option is selected. It seems that the client doesn't send the token for activating the clipboard.
This bug appear at the versions 2.3.3 after the commit r19895
r21377 should fix that. (strange how I didn't spot that earlier - could be another bug)
@cdrcnm: does that work for you?
Yes, I tested it and it works.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2110