Comment 76 for bug 362875

Reporting in from 16.04LTS, this bug still exists and Bug #1388416 appears to be describing the same bug and is marked Confirmed. I just signed up to post this so I'm still unfamiliar with the platform to find a way to link to it here.

I have the same issue, though it appeared to change behaviors after a recent restart-required update. Before the behavior was that after dropping, hot replugging resolved the issue 90% of the time; after the recent update hot replugging does nothing. At first I fell back to rebooting every time and then dug around for a bit and eventually found a more palatable work-around.

Issuing a restart command to the network-manager service often works, though you may need to restart your browser. Sometimes rfkill likes to soft-block the device especially after hot replugging so if it still isn't working try checking there.

I did notice the errors issued by ieee80211/rt2x00usb would be a quantity of 110s, then two or three 71s, and finally one or two 19s. This leads me to believe that something is hanging in the works or maybe the supplicant, that leads to the protocol error and eventual bailing out at the end. I'm not entirely qualified to guess, but I'd say it's possibly an issue in the kernel or systemd, but maybe network-manager. I doubt the module itself is to blame (since it worked in the past), but this is all my own conjecture and I have zero visibility deeper than the logs (I'd poke the guts but I moved from Arch to Ubuntu to avoid this kind of thing honestly). Incidentally back on 2.6.x kernel contemporary with the initial bug report up top, this problem existed on Arch so it perhaps is a chronic kernel issue.

The entities at play seem to be ieee80211, rt2x00usb, rt73usb, the kernel, and network-manager in some capacity.

lsusb:
Bus 002 Device 005: ID 148f:9021 Ralink Technology, Corp. RT2501USB Wireless Adapter

uname -r:
4.4.0-66-generic

dmesg (error 110 has dozens of entries above this and a few forced reset messages):
[27263.243242] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x06 failed for offset 0x308c with error -110
[27263.347192] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x07 failed for offset 0x3080 with error -110
[27263.451185] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x06 failed for offset 0x3080 with error -110
[27263.555188] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x07 failed for offset 0x308c with error -110
[27263.659189] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x06 failed for offset 0x308c with error -110
[27263.763191] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x07 failed for offset 0x308c with error -110
[27263.867203] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x07 failed for offset 0x308c with error -110
[27263.971195] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x06 failed for offset 0x308c with error -110
[27264.075198] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x07 failed for offset 0x308c with error -110
[27264.175843] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x07 failed for offset 0x308c with error -71
[27264.277826] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x06 failed for offset 0x308c with error -71
[27264.375643] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x07 failed for offset 0x308c with error -71
[27264.426576] ieee80211 phy2: rt2x00usb_vendor_request: Error - Vendor Request 0x07 failed for offset 0x308c with error -19

More dmesg, hot replugging only makes it worse:
[37659.214102] ieee80211 phy4: rt2x00usb_watchdog_tx_dma: Warning - TX queue 2 DMA timed out, invoke forced forced reset
[37661.189389] ieee80211 phy4: rt2x00usb_watchdog_tx_dma: Warning - TX queue 2 DMA timed out, invoke forced forced reset
[37662.201909] ieee80211 phy4: rt2x00usb_watchdog_tx_dma: Warning - TX queue 2 DMA timed out, invoke forced forced reset
[37663.277391] ieee80211 phy4: rt2x00queue_flush_queue: Warning - Queue 2 failed to flush
[37663.809381] ieee80211 phy4: rt2x00queue_flush_queue: Warning - Queue 2 failed to flush
[37664.189420] ieee80211 phy4: rt2x00usb_watchdog_tx_dma: Warning - TX queue 2 DMA timed out, invoke forced forced reset
[37664.673412] ieee80211 phy4: rt2x00queue_flush_queue: Warning - Queue 2 failed to flush
[37665.605427] ieee80211 phy4: rt2x00queue_flush_queue: Warning - Queue 2 failed to flush
[37666.201435] ieee80211 phy4: rt2x00queue_flush_queue: Warning - Queue 2 failed to flush
[37666.745441] ieee80211 phy4: rt2x00queue_flush_queue: Warning - Queue 2 failed to flush
[37667.293451] ieee80211 phy4: rt2x00queue_flush_queue: Warning - Queue 2 failed to flush
[37667.841452] ieee80211 phy4: rt2x00queue_flush_queue: Warning - Queue 2 failed to flush
[37668.189458] ieee80211 phy4: rt2x00usb_watchdog_tx_dma: Warning - TX queue 2 DMA timed out, invoke forced forced reset