Hi @Hippo1, I am sorry to hear about this.
As there is no way to adjust the functionality of the navigation keys using the settings in the phone, I suggest that you try to repair the software on the phone using a computer and Xperia Companion. Keep in mind that a software repair will erase all user data from the phone, so it is advised to do a backup of your user data before proceeding with the repair.
If the software repair do not manage to improve the functionality of the navigation keys, I suggest that you get in contact with your local support team. They can help you to have your phone sent to an authorized service center to be physically examined. If you have any questions regarding the service, such as routines, warranty and consumer laws, these type of questions are best answered by your local support team as well, as they vary between markets.
I hope that helped!
