The full error message may read: "The application has experienced a system error. The reported error message was: Unable to determine the data headers for the record type merge field."
This error message comes about because for a particular member, it does not appear as though they have the merge field that you have setup that defines if a record is a constituent or an individual. This is probably because the merge fields have changed somehow.
The best way to resolve this is to go to the Chimpegration Classic settings for and check that the value is still valid for the audience (our sample is listed as "Relationships Test") as shown below.
