Why does HTC Incredible S on Android 4 not always switch the screen on?

I have a HTC Incredible S shich was recently updated to Android 4 and Sense 3.6. After that, it began to not always switch the screen on after pressing the power button. In 20% of the cases, it just turns the backlight of both the screen and the buttons on, but doesn't "turn on" the image on the screen, so it stays black. Pressing power button 2 times (1 to switch off again and 1 to switch on) does help, but it's annoying. What can be the reason? How do I troubleshoot it? My phone isn't rooted.