Check if your web.config file has this line under  episerver.baseLibrary/classFactories:

<add type="EPiServer.Core.PropertyControlClassFactory, EPiServer" id="PropertyControlFactory">

Also, in what stage of Global.asax do you try to do the registration? It could be that it is done too early.

Maybe this is the same as register a Property ControlAdapters in the App_Browsers folder

In what method does this set up reside? Could it be that this code is executed before the PropertyControlClassFactory has been set up? Perhaps you could move it to a later event in the application start up?

Thanks for your suggestions.

The registration is under Application_Start event within global.asax

<add type="EPiServer.Core.PropertyControlClassFactory, EPiServer" id="PropertyControlFactory"> exists in web.config.

We have a site working correctly on a staging environment that was handed over to us.  The code and configuration is on the development environment is virutally the same as stage.  The main differences are: location of the EPiServer database and OS.  Stage is on a Server 2008 and development on Windows 7.  Also the solution is using EPiServer Mail.  Could that be tripping the code up>

Application_Start could be too early so it could be down to timing which can give different behaviour on different environments. I suggest you implement an InitializableModule and move your code to its Initialize method.

I installed the site onto Windows Server 2008 which had EPiServer 6 CMS on it.  The site then ran successfully.  So I installed EPiServer Mail 5 but that broke the site.  Removed EPiServer Mail but left framework 6.1.79.0 and now getting the PropertyControlFactory error.

If you just want a quick solution move this configuration to the configuration section in web.config. You should have a commented out section with an example on how to do this in the default installation.