I found way to fix this bug.
Problem was found not in driver VBoxSF.sys, but in system driver rdbss.sys (filesystem provider helper).
Sometimes it access to shared folder driver as named pipe in Win 7 x64. It happens very rarely.
I try to apply some fix strategies, but only one works fine. When VBoxSF.sys reveive a request typed NET_ROOT_PIPE, it manually changing to NET_ROOT_WILD in function VBoxMRxCreateVNetRoot.
And this is the interesting part. It would be nice if this change fixes the behavior but we need to understand why this translation is necessary.
