So I'm not yet able to pin-point the problem. Once I cured the symptom
by killing the two '/usr/bin/VBoxClient --clipboard' processes and
starting one instance manually. But that is of course just a hack. The
trouble supposedly started after I updated my Xubuntu guest OS.
typing VBoxClient --clipboard seemed to fix it, although it's not clear how because it didn't kill the old processes. But, that's a huge help, because at least I can copy/paste within the VM. Previous workaround was using the center-click in X in the VM, which does not seem to get copied to Sculpt (although center-click in Sculpt seems to paste the regular clipboard - probably Sculpt only has one clipboard unlike X).
If you have further observations or even a way to reliably trigger the
problem, I'd very much appreciate it.
I'll check on this next time I reboot, but I've never "not had" this problem until now as far as I can remember, so probably my Debian install triggers it pretty reliably. I'll confirm that when I restart, because for now it's solved. Two quick observations - copying more than 814 lines of sequential 3-digit decimal (i.e. 4 bytes a line in UTF-8) gives "Error: could not write clipboard data." from vbox, but seems to work within the VM (now). It should be noted that within qt5_textedit the limit appears to be 1018 lines, and within the Sculpt "Files" editor itself much larger copies are possible (it's difficult to tell because that editor is not user-friendly when it comes to large files). The VM limit is ballpark for what I observed before in terms of in-VM copy failures, but I'll have to double-check when the issue resurfaces. Second observation, sometimes, for unknown reason, there will be a group of "Error: invalid clipboard xml syntax" messages from vbox. It seems to happen when the Sculpt editor is used.
BTW, if you set the 'verbose="yes"' attribute in the clipboard
configuration in Sculpt's config fs, you can observe all copy operation
that arrive at the global clipboard in the log.
Helpful, I have turned this on, thanks.