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Hi hesper
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Since this has occurred after the update what I suggest is running a Software Repair on your handset as it is very likely to fix the problem.
Download the PC Companion software and install it on your PC.
Once installed on your PC, double-click the PC Companion icon on your desktop to start the update.
Please ensure your phone is powered off and not connected to the PC when you begin this process. You are not required to connect the phone until after step 10.
1. Install and open PC Companion then select Support Zone.
2. Phone Software Update.
3. Start.
4. Repair Phone
5. Continue
6. Accept data removal
7. Next
8. Wait for prepare
9. Select Phone
10. Follow the connection steps
NOTE: The software repair will delete all user data
To back up your phone using PC Companion:
1. Go to Support to install PC Companion on a Windows computer.
2. When installed, start PC Companion on the computer.
3. Connect your phone to the computer using a USB cable.
4. Click Start under Backup & Restore.
5. Follow the on-screen instructions.
Hope this helps! Keep us updated 
Dears,
I'm having exactly the same problem, the locked screen is not totally black out. As if there's power coming out of the black screen.
Is this a softwware or hardware issue? I don't know.
Appreciate your support
Regards,
Ahmed