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Camera used by another application
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Hi @Phil221, let me see if I can be of any help here.
Your question has been translated 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.
You might experience that the message "Camera used by another app. Close the other app and try again." appears when you open the camera app on your Xperia device. Sometimes an app can be using the camera in the background, and it may be difficult to identify which app to close. Therefore you should try restarting your device as a restart will clear all background processes in the phone.
Try this and let me know how it goes! 
I have the same Problem and i started it 2x again and still won't work
Hi @eddy5,
In that case I suggest that you try to perform a software repair using a computer and Xperia Companion. Please note though that a software repair will erase all user data, so remember to do a backup of your user data first.
Try this and let me know how it goes!