Hi @Dutertz
Welcome to the community!
First thing to do would be check if there are any software updates available for your handset.
To do this, first of all, if it is not already installed, download and install the Xperia Companion application from http://support.sonymobile.com/global-en/xperia-companion/.
You can then select the Software Update option which will check if there are available updates you may not have previously installed, it will then prompt you through the steps of downloading and installing the update if there is one.
If there are no updates available, next I would advise removing the SIM card and/or SD card if you have one in the handset. Run it without these installed to see if the issue still occurs.
Let me know how it goes 
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