When adding a WFS service to Map Viewer via service URL, you may receive the error message "Error while parsing the response". Map Viewer makes GetFeature requests to the WFS service endpoint to return the geometry and attributes for all the features in the service.
If the response to these requests is improperly formatted, an error message occurs.
If this issue occurs, a good next step is to collect network traffic while reproducing the issue. In the Console tab, there may be an error about "malformed JSON". Check the responses from the requests to the WFS service that contain the parameter "OUTPUTFORMAT=GEOJSON". Copy the JSON from the responses and analyze them using JSONLint. This determines if the JSON returned by the WFS service is valid or not.
In some cases, the JSON may be improperly formatted due to null, missing, or incorrect geometry.