Suggested fix from licensee:
After investigation, it seems that the filename is passed as a string instead of a FileReference in Script.cs:84.
Fixing the issue seems to be as simple as changing the line 84 of Script.cs from:
Log.TraceError(File.FullName, Ex.LineNumber, "{0}", Ex.Message);
to
Log.TraceError(File, Ex.LineNumber, "{0}", Ex.Message);
Results:
The error message when there is an error in a buildgraph xml file is just "ERROR: " instead of the full information.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-79647 in the post.