My experience with "VM Hypervisor failed" has been:
- It can happen when BOINC is started while an old VBoxSvc.exe is already running. For instance, just last night, I shutdown Windows, but because Fast Boot is enabled on Win8/10, it basically just logged out and hibernated. When I turned the PC on, and it resumed from hibernation, VBoxSvc.exe was still running. When BOINC started, it tried to use it, and failed. Recovery involves restarting the PC, and then the task will usually work.
- It can happen when a task is corrupted. This happens when Windows abruptly stops, for instance, when Windows gets a Blue Screen of Death, or if there is a sudden power outage. Recovery involves using a previous manual backup of the VM, and careful offline data manipulation and testing, to get BOINC to work with it again. Basically, not recoverable, unless you are an expert that was taking VM backups.
So, if you have the problem, try restarting the PC. It might make the task work again. If it doesn't, then the task is likely broken.
PS: Whenever I need to reboot the PC (to install a new Windows 10 Insider Build for instance) or whenever I need to install software, I *always* perform these steps:
- BOINC -> Activity -> Suspend
- Watch Task Manager, and wait patiently for the BOINC-related processes to gracefully exit, including boinc.exe. This can take up to a minute.
- After I am completely positive that all BOINC processes have gracefully quit, manually kill VBoxSvc.exe
- Install my software
- Reboot the PC, if necessary
- Start BOINC
- After BOINC starts, BOINC -> Activity -> Run based on preferences
Basically, I don't let Windows try to close/kill BOINC on a restart. I don't trust it. Instead, I make sure to use my steps for a definite graceful shutdown.
PS: René, it looks like the task did fail for you, losing 2 months of work. Sorry for your loss. If it's any consolation, I was your wingman on that task. My PC "Nitro" has been working on it since 6/5/2015, and I vow to work even harder to complete that monster!