The document specification <ddlname.schemaName+SchemaRootNode> from assembly <etc…> failed to load. Verify the schema for this document specification is deployed and is in the Global Assembly Cache.
In our case, this happened when we ran a C# client tester program that called an orchestration published as a WCF service. The Application Pool was not configured correctly, and had .NET v2 instead of .NET v4. Thus it was looking in the “old GAC”, rather than the new one. The schema was truly deployed and GAC’ed. We just changed the app pool and restarted it.
This happened to me again 05/19/2021. Just go back to the basics. The “Receive Location” runs under a “Host Instance”, and that “Host Instance” has to be restarted. I modified a schema/project, and I’ve been accustomed to just restarting the Orchestration Host Instance, but I have to restart the Receive Host Instance as well (anytime a receive schema changes).