[Zubersoft]

I'm really not sure what you are referring to with register devices with Dropbox. I'm currently using Dropbox's library to access their service, and the login process returns back something called an oauth2 token. This token allows for access to the account without having to enter the login information every time. The token has an expiration date though so eventually you still have to log back in at some point. I'm not aware of any per-device registration with Dropbox unless I'm missing something. Can you show me where you are seeing the per-device registration information in the account?

As far as the errors you are encountering while synchronizing, I'm not sure what is going on there as I'm not sure how you have set up the sharing perimssions for your folders. I have certainly tested synchronizing to a folder owned by a different user without errors, but I did not try uploading to that folder though, so that may be why I didn't see any issues. 

Mike