Error in custom activity: Could not load file or assembly 'Microsoft.IdentityManagement.WFExtensionInterfaces'

I had to roll back an update which was causing trouble, and rever to FIM version 4.1.3510.0.

Things look ok, but workflows with custom activities are ending in PostProcessingError, and I see this in the event log:

"Forefront Identity Management Service is not able to serialize this XOML definition"

Checking further in the event log, I see

"System.Workflow.ComponentModel: System.Workflow.ComponentModel.Serialization.WorkflowMarkupSerializationException: Could not load file or assembly 'Microsoft.IdentityManagement.WFExtensionInterfaces, Version=4.1.3510.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies."

However, if I check in C:\Windows\Assembly, that DLL is actually there, with the proper version.

Any hints?

Thanks in advance,
Paolo 

Paolo Tedesco - http://cern.ch/idm