Couldn't retrieve child elements from the server. Getting this error when want to getfolders at 2 level deep ("/folders_level1/folders_level2") from Root.

It looks like this error message is coming from the SharpBox library, and not the Dropbox API itself. As it's made by a third party, we can't provide any support for it of course. If you can print out the error being returned by the Dropbox API though I should be able to offer more guidance. (That is, the Dropbox API returns an error code and message for an API call that fails that should offer some more useful information, but SharpBox doesn't seem to be exposing those.)