Constant loss of connection

[Nachteule]

I suppose you're using the onboard wlan?

Unfortunately this broadcom wlan is absolutely crap, having similar issues here:

Sometimes wlan is working well for weeks w/o any issues, and from one day to another having same issue you described. No idea how to solve it, already used different firmwares with no success.

In my case after a while wlan is back, maybe you just have to wait. Strange thing is, my server can connect to rpi3 mostly, my notebook can't.

Which kernel do you use. If you're running 4.9.x, you could revert to 4.4 to see if there is any difference