Regarding debugging, since you have external db connector installed, you should see a Log Node by the same name. You can find this in your Console View > Advanced > Set log levels > external database connector > set it to Trace.
It should give you outputs like below
executeQuery: jdbc:postgresql://localhost:5432/testdb, root, select * from employees
Getting connection from data source in connection pool for [url=jdbc:postgresql://localhost:5432/testdb, user=root]
Result list count: 3
See if you can spot something abnormal there. This should reveal enough information for you to find the RCA.