I have the same problem as SlrG; when starting the vboxwebsrv from the unRAID webui, it stalls the webui, and makes it unusable, even when refreshed, it times out completely. The unMenu webui still works fine, below is the syslog from when i was working with this, i started the vboxwebsrv a few times, and killed it via a telnet session in order to get the unRAID webui to respond again. I haven't installed any VMs yet in virtualbox either.
Mar 19 16:41:28 ShadowOfIntent kernel: md: recovery thread woken up ...
Mar 19 16:41:28 ShadowOfIntent kernel: md: recovery thread checking parity...
Mar 19 16:41:28 ShadowOfIntent kernel: md: using 1536k window, over a total of 2930266532 blocks.
Mar 19 20:51:28 ShadowOfIntent rc.virtualbox[9412]: Plugin configuration written
Mar 19 20:51:39 ShadowOfIntent rc.virtualbox[9684]: VBoxManage setproperty websrvauthlibrary null
Mar 19 20:51:39 ShadowOfIntent rc.virtualbox[9699]: vboxwebsrv service started
Mar 19 21:05:31 ShadowOfIntent rc.virtualbox[10770]: No Virtual Machines running - nothing to save
Mar 19 21:05:31 ShadowOfIntent rc.virtualbox[10772]: vboxwebsrv service stopped
Mar 19 21:16:36 ShadowOfIntent rc.virtualbox[17150]: Symbolically linked /boot/config/plugins/virtualbox to /root/.VirtualBox
Mar 19 21:16:36 ShadowOfIntent rc.virtualbox[17153]: Moved existing files from /boot/custom/vbox (and removed folder) to /boot/config/plugins/virtualbox
Mar 19 21:16:36 ShadowOfIntent rc.virtualbox[17154]: Plugin configuration written
Mar 19 21:18:36 ShadowOfIntent rc.virtualbox[18417]: Plugin configuration written
Mar 19 21:21:50 ShadowOfIntent rc.virtualbox[20385]: VBoxManage setproperty websrvauthlibrary null
Mar 19 21:21:50 ShadowOfIntent rc.virtualbox[20400]: vboxwebsrv service started
Mar 19 21:22:23 ShadowOfIntent rc.virtualbox[20430]: No Virtual Machines running - nothing to save
Mar 19 21:22:23 ShadowOfIntent rc.virtualbox[20432]: vboxwebsrv service stopped
Mar 19 21:30:39 ShadowOfIntent rc.virtualbox[25645]: VBoxManage setproperty websrvauthlibrary null
Mar 19 21:30:39 ShadowOfIntent rc.virtualbox[25660]: vboxwebsrv service started
Mar 19 21:38:52 ShadowOfIntent rc.virtualbox[26328]: No Virtual Machines running - nothing to save
Mar 19 21:38:52 ShadowOfIntent rc.virtualbox[26330]: vboxwebsrv service stopped
Mar 19 21:38:59 ShadowOfIntent rc.virtualbox[26619]: VBoxManage setproperty websrvauthlibrary null
Mar 19 21:38:59 ShadowOfIntent rc.virtualbox[26634]: vboxwebsrv service started
Mar 19 21:38:59 ShadowOfIntent rc.virtualbox[26715]: No Virtual Machines in previous session (list is empty)