Had a look at this, but as it is a piece out of a much more complex model it's impossible to do much.
Is the Site BO a member of a Business Object Group?
If it is then you get this error when you have an attribute type mismatch between two BOs
For example
BOG Vehicles has BO Cars and BO Trucks and have common Attribute Wheels.
If Cars.Wheels is of type Text but Trucks.Wheels is Number then this error will occur whenever you are trying to make updates. It is not a helpful error message and I only figured this out by accident.
I can't help you diagnose it though, you'd need to look at the BOG and check every member and every common attribute.