Android 6 Nexus 5 USB Connection problems

had the same problem, so i enabled developers options, went in the line for usb configuration. it was on "MTP" so i switched in another mode then came again on MTP and it worked. Hope it will work for you

I have a nexus 9, and Android 6.0. You have to go into the developer options. Then under networking, you select USB. Eventhough it is already selected as MTP, just press the MTP option again. It is frustrating that you have to repeat this every time you reconnect the device.