Could you please try re-saving the database rule and segment and then checkin the segment (this will regenerate the report) and then try with "show customer" option and see if it works. Else would request you to capture the tracer and share the same
I did a search for you and found out the possible root causes:
Please check if there's an invalid mapping on the external mapping tab for the customer class. In some case, mapped the partition key property is missed. If it's not obvious from the mappings tab, try creating a report def on the customer class and pulling in properties from that class, that'll show you the property or properties that are causing the problem.
In another case, the issue was related with mismatch between CustomerID data type. In Pega Marketing version 7.1.3 it should be test / varchar2 but users mapped it as Number. Thus, report definition failed with exceptions.