Hyper-v logoff / logon issue

When running through final preparations for a presentation to a few hundred people (well, tens of people…ok, at least 5 people…proabably) the last thing you need is for your Hyper-v environment to start misbehaving.   This is exactly what happened to me and I’ve just found the cause, so am feeling a lot more relaxed about things.

 I had prepared a number of Hyper-v VMs (child partitions, if using the Microsoft jargon) and found I was not able to log on to any of them following a log off.  The VMs were a mix of Windows Server 20008 64bit and 32bit.   When trying to re-log on, I received the error “Your system is running low on resources.  You cannot log on as a new user.  Please use an account that has already been logged on.”

The only workaround I found for this scenario was to shut down the machine and boot up again. 

After investigation the error about “running low on resources” appeared to be spurious.  The VMs were running fine and weren’t under any real resource load.  Assigning more memory and/or additional processors made no difference.  The problem was 100% reproducible.

 In the end I noticed that a newly created Windows Server 2008 VM did not exhibit the same problem.  From there it was a question of trial and error, running through each step of my build process to see what introduced the problem.  Bizarrely, it turned to be changing the default 96 DPI font size to large font (120 DPI).  I had been changing to the bigger font to save my failing eyesight! 

 As soon as I changed the font back to 96 DPI on my VMs the problem disappeared.

 I’m now trying to find out whether this is a bug, or just something weird in my environment.