If I had to guess, there was some cached information that was updated by you reselecting the folder. Perhaps the oauth2 token had expired and Dropbox was requiring another one, but just starting the sync wasn't sufficient for this. I'll have to investigate that to see if I can better detect that scenario. The first thing the code does when you start a sync to a cloud folder is go through the cloud service initialization which does a quick API call to make sure the token is valid. The question is why that didn't trigger reauthentication for you if the token was expired. If it's not a token issue, then I have no idea why reselecting the folder would have had an impact unless the original saved folder path was incorrect.
Mike
Mike