on your Linux guests you could stop the "VBoxClient --clipboard" process and
restart it (as your user in your X session) as
"VBoxClient --clipboard --nodaemon".

You can also spawn an extra one of these, if you have simultaneous X sessions
in the guest and clipboard transfer works in one (usually the first one) but
not the others.

to debug, set the environment variables
"VBOX_RELEASE_LOG_FLAGS='timeprog thread'"

This will produce a similar log file called vbox.log for the guest in the
current directory. When you are finished you can stop VBoxClient --clipboard
again and restart it without the --nodaemon parameter, so that it goes back
into the background.