This is how it goes some days. I've been looking at this for two hours, and five minutes after I post this question, I tried something that worked like a charm. The key is to declare MeasureSummary as a known type for the service.

Code:
[ServiceContract, ServiceKnownType(typeof(MeasureSummary))]
With that in place, things are working again. Sorry to bother you all, and have a good day.