I still experienced a similar error due to too many connections. This error came too much and asked everytime for the password - the username is still filled in, the password is stored in keychain. It got nasty, so I aborted, then the transfer was marked as completed, but it wasn't. Now I uploaded again the last file, which was incomplete, but I could not resume. I needed to overwrite, because I had the option Overwrite or Rename. This is bad for long files.