With the latest SDK Update (6.0.4) Microsoft quietly removed support for .Net 4.5 in Custom Workflow Activities.
This is a rather unexpected move as it is still supported for CRM 2011 CWA’s.
And here is where the problems start: If you have .Net 4.5 Custom Workflow Activities deployed in a CRM 2011 environment – which is supported – and then run an upgrade to CRM 2013, suddenly they are unsupported.
As we do currently not have more information on the specific issues regarding .Net 4.5, we cannot say what that means in detail and where the problems are.
So to be fully supported (whatever that means) we have to redeploy these Custom Workflow Activities using .Net 4.