Just an update on this, it kept happening until I stumbled on another thread advising to update the login in growattServer/init.py to "newTwoLoginApi.do" rather than "newLoginAPI.do". Seems they changed this in the latest update.
Many thanks Mike for sharing your script, it was almost exactly what I was looking for. It was all working fine, as was another script I wrote, until the other day and now it seems to error on the login part. Is anyone else having these issues?