I’ve heard of this issue and I think that, unfortunately, this indicates a hardware failure, since you couldn’t solve it by deleting WPA_supplicant… 
(Read more here)
Anyway, you might try a hard reset to rule out software errors and if this doesn’t help, then you should contact Fairphone support and follow their instructions (you can link to this thread).
PS: Support may take over a month to answer, so if it’s urgent you could buy a WiFi daugtherboard from the shop and try to repair the phone yourself. In any case tell us what you did to solve the problem! 
Edit: Read the post below please. Replacing the daugtherboard might not resolve the issue.