There is freezing issue for update lollipop 5.1 for Z Ultra. Whenever I use any video player, once I scan to the desired portion of the video, everything freezes and needs for me to close the video player or do a soft reset.
I have a tablet z on 5.1 and z ultra on 5.0 The tablet z does have problems playing videos through browsers. The z ultra works fine. I kind of suspect the problem kicked in after the 5.1 update.
I think the problem is just not serious enough to justify a factory reset, where I have to set up everything from scratch. Even the back up restore thing can't bring back everything you know.