Calculate and Store for a parent is not working

Summary:

I've a use-case for which my source is a list viewpoint and the target is a hierarchy viewpoint. I need to automatically place the source nodes under the designated target nodes in the Target Viewpoint.

1. Created a node type converter from Source to Target. Returning <unknown> for the Parent.
2. Copied the property carrying the parent value to another property (say, xyz) in the target node type
3. Using Calculate and Store for the Parent node type; Returning "xyz" as the value.

This worked perfectly fine until last week (in the production instance) but is currently not working. I'm getting the error - "