Getting "ConstantContactServiceException: Response came with errors from WebService. Please see error info." but [exception].getErrorInfo() is null. What to do next?
Further information - the uploaded file had one contact in it. When we added two more contacts, the error stopped occurring. Is there a minimum number of contacts that must be in a bulk upload file?
When you are uploading a file to the API, there should be no limit on the number of contacts, as long as it is properly CSV formatted with only one contact. The first thing I would suggest double-checking is that the formatting of the single line of data is handled the same way as when you are submitting multiple contacts.
In addition, you can try debugging against the RestClient class that is implemented in the SDK so that you can check the HTTP response directly. This may have a varying level of effect, as the API can return errors that are less useful when it is unable to parse an uploaded file(due to formatting) rather than having invalid data in an uploaded file.