Google Nexus 5 — No Wifi connection since I installed Marshmallow

My Nexus 5 detects all available wifi networks OK, but will not connect to any. The messages are varied - an exclamation mark ! next to the wifi signal, "authentification problem", but the end-result is the same "no internet".
I tried outside of my own home. at friends', in the city nearby, - same result.
My laptop finds the home networks OK.
I stopped and restarted my modem several times.
I did 4 factory resets.
No result.
Thank you for any suggestion, besides waiting for Google to fix the problem...