Windows 8.1 Update and 2012R2 update were released last week.
unfortunately installing the update on a Hyper-v 2012 R2 host broke Veeam Backups.
Veeam have been extremely quick to release a patch to fix the issue.
You can find the patch along with an explanation regarding the problem over here at Veeams site.
The update is highly recommended for Hyper-V hosts as it includes many patches for failover clustering that we always recommended installing.
So if you’re using Veeam go ahead and update you Veeam backup software and then deploy the update to your hyper-v hosts.