Can't start up client

[mother]

Sometime when I have boot up a client for a long time, say 2 days.
I can't boot up any of client (win7 and centos 7)and the follow error code show up.
And I cannot boot up any of clients afterward until I restart the win 7 host.

Result Code:
RPC_S_SERVER_UNAVAILABLE 0x800706BA (0x800706BA)
Component:
ProgressProxy
Interface:
IProgress {c20238e4-3221-4d3f-8891-81ce92d9f913}

can I stop some service or do anything instead of restart the host? Because I have to shut the rest of client before reboot

[mpack]

Offtopic. There is no VBox version currently in beta.

Please post in the proper forum and provide Minimum information needed for assistance.