I am running FreeBSD 11.3 in a VirtualBox guest with shared clipboard set to bidirectional. But the the shared clipboard did not work.
I found the reason for this - the shell script /usr/local/bin/VBoxClient-all
tries to invoke VBoxClient from /usr/bin (instead of /usr/local/bin) at line 38:
/usr/bin/VBoxClient --vmsvga-x11 # In case VMSVGA emulation is enabled
Of course, there is no /usr/bin/VBoxClient