I have been using a Hyper-V 2012 R2 host with multiple VMs in it (mainly Windows 7 and Server 2008 R2) with no problems for a long time. Installed a new Server 2012 R2 VM and created a new VHDx to store around 500GB of data. When starting to move files from a physical machine to this VM, all VMs were losing network connectivity. Looking at the network connections these looked OK, no red Xs or similar on the VMs or the actual NIC, but couldn’t communicate with any VM unless I disable and enable the NIC back.

Microsoft KB2986895 explains the problem and after disabling VMQ on the NIC used by the Virtual Switch the problem was solved (Did it through the Advanced Properties of the NIC).