Hi @Maria1979, let me see if I can help out here!
I have translated your question to English as this is an International forum, please post any replies in English and it will be easier for me and other users to assist you.
The error message saying that "System UI has stopped is usually generated by a downloaded app in the phone. Since it might be hard to detect which app in the phone that is generating the error message, the easiest way to get rid of the error message is to repair the software using a computer and Xperia Companion.
Keep in mind though that a software repair will erase all user data from the phone, so make sure to do a backup of your user data before the repair.
I hope that helped! 
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