I have now moved from the RPMFusion build to the RPM one on VirtualBox website, and the same thing happens.
In this case the VM has been running quiescently for a few hours, but when I came to use it again it crashed out.
$ VirtualBox --startvm "Windows 7"
Qt WARNING: libpng warning: iCCP: known incorrect sRGB profile [repeated several times]
VirtualBox: xcb_conn.c:186: write_vec: Assertion `!c->out.queue_len' failed.
Aborted (core dumped)
Again, log file doesn't show anything of great interest (attached).
I got a core dump from this, and the backtrace is:
#0 0x00000033e7235877 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#1 0x00000033e7236f68 in __GI_abort () at abort.c:89
#2 0x00000033e722e7d6 in __assert_fail_base (fmt=0x33e737bb48 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n",
assertion=assertion@entry=0x33ea2176eb "!c->out.queue_len", file=file@entry=0x33ea2176e0 "xcb_conn.c", line=line@entry=186,
function=function@entry=0x33ea217704 "write_vec") at assert.c:92
#3 0x00000033e722e882 in __GI___assert_fail (assertion=0x33ea2176eb "!c->out.queue_len", file=0x33ea2176e0 "xcb_conn.c", line=186,
function=0x33ea217704 "write_vec") at assert.c:101
#4 0x00000033ea20a183 in _xcb_conn_wait () from /lib64/libxcb.so.1
#5 0x00000033ea20a4c3 in _xcb_out_send () from /lib64/libxcb.so.1
#6 0x00000033ea20ac57 in _xcb_out_flush_to () from /lib64/libxcb.so.1
#7 0x00000033ea20acf8 in xcb_take_socket () from /lib64/libxcb.so.1
#8 0x00000033e9e42536 in require_socket (dpy=0x7f77587e11e0) at xcb_io.c:68
#9 0x00000033e9e42db9 in _XFlush (dpy=0x7f77587e11e0) at xcb_io.c:511
#10 0x00000033e9e45805 in _XGetRequest (dpy=dpy@entry=0x7f77587e11e0, type=type@entry=43 '+', len=len@entry=4) at XlibInt.c:1735
#11 0x00000033e9e3ea1b in XSync (dpy=dpy@entry=0x7f77587e11e0, discard=discard@entry=0) at Sync.c:43
#12 0x00000033f9848cda in dri2XcbSwapBuffers (pdraw=0x7f772c926fb0, remainder=<optimized out>, divisor=0, target_msc=0, dpy=0x7f77587e11e0)
at dri2_glx.c:817
#13 dri2SwapBuffers (pdraw=0x7f772c926fb0, target_msc=0, divisor=0, remainder=<optimized out>, flush=<optimized out>) at dri2_glx.c:855
#14 0x00007f7737ba3a72 in renderspuVBoxPresentCompositionGeneric () from /usr/lib/virtualbox/VBoxOGLrenderspu.so
#15 0x00007f7737ba6130 in renderspuWinCmdThreadProc () from /usr/lib/virtualbox/VBoxOGLrenderspu.so
#16 0x0000003cecef575c in rtThreadMain () from /usr/lib/virtualbox/VBoxRT.so
#17 0x0000003cecf6ad5c in rtThreadNativeMain(void*) () from /usr/lib/virtualbox/VBoxRT.so
#18 0x00000033e7a07ee5 in start_thread (arg=0x7f7760167700) at pthread_create.c:309
#19 0x00000033e72f4b8d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111