I've upgraded from 1.7.1 to 1.7.7.1 just to found out a missuse of cookie->write() at Context->updateCustomer(). It should be cookie->update(), as this causes to have a cookie twice.
I found this because I've a custom REST interface into my PS for a really old software, so when receives 2 cookies, it doesn't process the server response.
I had this problem in the past because a paid module made the same.
Please, developers, don't use cookie->write() everywere, as it's written automatically on _destruct event