Problem: While running PowerPoint on a Windows 7 guest (starting to see a pattern here?), the virtual machine crashed repeatedly when using the clipboard.
There was no trace of the crash in the VirtualBox logs and journalctl was also very brief:
$ journalctl -xe | grep Virtualbox
[...] audit[74104]: ANOM_ABEND auid=1000 uid=1000 gid=1000 ses=3 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 pid=74104 comm="SHCLX11" exe="/usr/lib/virtualbox/VirtualBoxVM" sig=6 res=1
Solution: Disable the shared clipboard until this issue is fixed.