Enable Azure VM remote debugging for pre-Azure SDK

The introduction of remote debugging for Azure really saved a lot of time, and it usually works fine - as long as your VMs or Cloud Services were created recently.

However, I've run into problems while trying to debug an older VM (which is in production, so just recreating it is not an option). I've done everything that is needed for the old VMs to support the VM extensions as described here, installed the remote debugger extension as described in this article, but after clicking "Attach Debugger..." I keep getting this error:

There was a failure to launch the remote debugger.

In the Azure SDK 2.2 release notes there is a similar entry under "Known Issues" which suggests to restart Visual Studio, but this didn't work for me, and it might be only relevant to cloud services.

Are there any known solutions/workarounds for this other than setting up a VPN and attaching an old good non-Azure remote debugger?

Additional info

I'm using Visual Studio 2013 Update 3 with Azure Tools 2.4 - if this makes any difference.
While looking for a solution I've run across another StackOverflow thread that describes a similar problem, but for debugging cloud services. I'm posting this question for two reasons:
1. Azure Cloud Services are configured differently to Azure VMs, so the solution (if there is any) may be quite different.
2. There is no relevant solution in that thread.

Have you verified that the Extension is installed on the VM. Easiest way I know is to right click the VM in the Azure Explorer and select "Configure" (like this-> http://msdn.microsoft.com/en-us/library/azure/dn569263.aspx#BK_ManageVMs ) . It'll show you a list of VM extensions installed.

Also, if you REMOTE into the Virtual Machine you should be able to see the "Log" files from any extensions that were installed (or failed to install). You'll find the log info (and other extension related components) here: C:\Packages\Plugins\Microsoft.VisualStudio.Azure.RemoteDebug.VS2013Debugger\