character "5" repeated in text fields, and system freezes
Hardware problem! Some piece of cruft is making the 5 key stick. Clean your keyboard or try another keyboard.
This is a bug and is already reported. It seams it has a fix also but I think you have to upgrade your kernel. Read this for more info. @KamalMostafa has provided a linnk to patched kernel in the bug's thread. I hope that the Ubuntu 16.04.1 will have the fix included.