Sometimes you need to look at the logs of the connector because the initial error is not very descriptive and you need a few more clues.
The logs are held in an unusual folder – ProgramData, which is often hidden from browsing, so you often have to paste the folder into windows explorer.
below are the steps to start the logging and where to find the file.
You may have problems editing the Microsoft.Dynamics.Integration.Service.exe.config file and the way around this is to copy the file to another folder, make the change and then copy it back and overwrite the original.
You can get this log following these steps: