Screen goes black

The last few weeks I notice a problem with my FP2. When I call a number (and a choose the right sim card), the screen of the phone goes black.
Normally I could use the powerbuttom to bring the screen back. But now that doesn’t work anymore.
This is very anoying. Because of this it is not possible to stop the call, pick the right number when there is choose list (voicemail), …

How can I fix this

See this thread: