My 87-yr-old partner and I have same phone. I am his "techie". I updated both of our phones to Lollipop. No problem with mine. Problems mentioned here with HIS. Tried fixes suggested in many forums to no avail!
Then compared settings on both of our phones to make sure they were the same. Still no luck, until removed the "Guest" User. How does one find the needle in this crazy hay stack?
Reading more of the posts below, I realized that I use a Screen Lock and he did not, so maybe that was the relevant difference.
In any case, today is my birthday, so thanks for the gift of this "correct answer".