Virtualbox Copy and Paste not working - it's Excel?!

Just thought I'd post here since I've found no other hits:

I've used Virtualbox (on Windows) for years and copying and pasting (of text, anyway) always seems to have gone through phases of working consistently well or not working at all.

Anyway, I'm now running a Windows 7 VM on a Windows 10 host and 've noticed that the thing that breaks copy and paste appears to be using Excel on the guest machine.

After breaking it, killing and restarting vboxtray appears to work for about one copy and paste (but maybe only in the host->guest direction).   
Rebooting (just) the guest doesn't seem to fix it - I think I need to reboot the host as well (which I tend to do infrequently).

I'm still running with the obvious solution at the moment: avoid using Excel on the guest.