I never thought of that, but it isn't at all surprising.I stumbled into an embarrassing solution. Turns out I had a tiny (13 mm) USB flash drive attached, but unmounted, and I had forgotten it. When I removed it, I could connect again.
I knew USB 3 attachments could interfere, but I would expect the interference to block any network. I wondered if the interference blocks 802.11b/g/n networks differently than 802.11ac networks? I did a bit of research and found a number of sites that said the USB3 interference is in the 2.4 GHz range, but not 5 GHz. Maybe I need a new 802.11ac capable router to protect myself from myself.
I can easily demonstrate severe interference between WiFi/Bluetooth and USB 3 devices - it's a very miserable problem (at least as bad as classic SCSI issues - worse, I think).
If I have a USB 3 storage device (USB 3 SSD) near the back of my 2015 MacBook Pro 15", it can make Bluetooth and WiFi quite flaky (to the point they don't work at all). Of course, it all depends on precise location, cable, device, etc., ad nauseum. I have to work around the problems - usually by repositioning the devices slightly.
802.11ac should be more robust than earlier wireless standards:
Your note prompted me to check my WiFi connection (Option-click on the WiFi item in the menubar), and I found I'd stupidly been using 2.4GHz, though I have an 801.11ac AirPort Extreme router.Wikipedia said:IEEE 802.11ac
The specification has multi-station throughput of at least 1 gigabit per second and single-link throughput of at least 500 megabits per second (500 Mbit/s). This is accomplished by extending the air-interface concepts embraced by 802.11n: wider RF bandwidth (up to 160 MHz), more MIMO spatial streams (up to eight), downlink multi-user MIMO (up to four clients), and high-density modulation (up to 256-QAM).
In June 2013, Apple announced that the new MacBook Air features 802.11ac wireless networking capabilities, later announcing in October 2013 that the MacBook Pro and Mac Pro also featured 802.11ac.
So, I opened up AirPort Utility on an iPhone and found that I could set a name specifically for the 5Ghz network, which I did. After making the change and waiting for the AirPort to restart, I manually connected to the new 5GHz network (new name) and then configured System Preferences > Network to put that 5GHz network at the top of my priority list.
Seems faster now...