Same thing here on Xubuntu 15.10 upgraded from 15.04 (where I had this annoying dialog occasionally).
There is a "solution" at http://mclements.net/blogWP/index.php/2015/12/11/stop-that-pesky-authorization-dialog/ that, according to my understanding, simply disables the dialog box. It seems to have the desired effect, but I do not understand why, and what side effects it causes, so I disabled it.
Same thing here on Xubuntu 15.10 upgraded from 15.04 (where I had this annoying dialog occasionally).
There is a "solution" at http:// mclements. net/blogWP/ index.php/ 2015/12/ 11/stop- that-pesky- authorization- dialog/ that, according to my understanding, simply disables the dialog box. It seems to have the desired effect, but I do not understand why, and what side effects it causes, so I disabled it.