Backups problem with the Dropbox feature

I have problems with the Dropbox backup settings.

Uploading archive to Dropbox ..
.. upload failed! JSON decoding failed : malformed JSON string, neither array, object, number, string or atom, at character offset 0 (before "(end of string)") at /usr/share/webmin/virtual-server/ line 184.

Anyone who have an idea for this.

Still no one?