Yesterday I’ve performed the upgrade of Veeam Backup and Recovery from version 126.96.36.199 (Patch 3) to version 188.8.131.52. After the installation is completed a reboot of the server is required. After initiating the reboot the VM keeps rebooting without the option to logon to Windows. I did a quick search in the Veeam knowledge base and the following article drew my attention; http://www.veeam.com/KB1645
This article describes exactly my issue. The problem occurs when you have the Veeam Enterprise Manager installed on the same system. Follow the instructions in this article prior updating B&R, it also describes what to do if you already experiencing the reboot loop.