Hi..again..,
I already send answer but then recieved the message that it got declared as "spam"
Short summary. I have the same problem, and I will try to replace the switch cap and see if that helps. Luckily I ordered the switch cap and the tools a few months ago, when I had trouble with my speakers.
But I guess that the source of the problem lies deeper.. have a look here:
https://forum.fairphone.com/t/fp1-power-button-does-not-work-anymore/10820/19
If the new switch cap doens't help, I will order a new switch and try to find a shop that can replace the old one and solder it to the mainboard.
Good luck!
Maja