CRM 4.0: Workflow error on wait condition for custom activity

Symptoms

• You wrote a custom Workflow Activity
• You created a Workflow that contains your custom activity and a wait condition
• You deployed and published the workflow and it "kind of" works, but when you try to open a running workflow you get the generic "An error has occurred. Try this action again." message.
• You enable tracing and see that there are some exceptions that vaguely mention serialization.

Probable Cause

• You have properties in your Workflow Activity class and the Workflow Process doesn't know how to serialize them. This seems to be a problem for all properties that are not persisted using the Dependency Property mechanism.

Solution

• Remove the properties, unregister and re-register the assembly and off you go.