My Fairphone2 has on start up: "Unfortunately, Fairphone Launcher 3 has stopped."

[paulakreuzer]

In that case the correct topic is actually the following one (there is a workaround provided):

Slim Case - Reboots and Button-Issues

I am also experiencing reboot issues with the new slim cover. When pressing the power button I regularly manage to jam it and it does not release. The continuously pressed power button then causes the phone to reboot. Swapping the power button with the camera button (which I rarely/never use) resolves this issue for me.

About the Launcher error message: Is it gone too?