I have just upgraded to 20.04 from 18.04, and now I cannot scan. The error message says the device failed to connect. The only devices attached are a webcam and the HP Envy 5640 printer-scanner.
The printer still works, although a new printer has been created which also does not connect. Sometimes the scanner asks me to select which of the two devices to use, but neither works for scanning, while one works for printing.
I downloaded HPLIP, but that also has the same problem: "Unable to open device hp:/usb/ENVY_5640_series?serial=TH7779Y0D305ZC."
Update: The problem remains. I cannot see a consistent pattern, but sometimes I am 'unable to open device' and sometimes it appears to work: the empty outline of a document appears on the screen, the printer says 'scanning' on its display panel, and you can hear the sensor moving as expected below the glass. But, at the point when the image would start to be drawn on the screen, the error message 'Failed to scan', 'Unable to start scan' appears; the printer continues to say 'scanning' for several more minutes.
When I connect with the printer using wifi and an older version of Ubuntu, I can scan perfectly well, if a bit slowly. So the problem is clearly with the upgrade from the previous LTS to the current one.
I assume there must be a way to capture the actual messages going from the 'Simple Scan' program to the printer, but I have looked in the obvious places and can't see how to do it. How can I narrow down the source of the problem?
21 Answer
The problem has been solved by updating to Ubuntu 20.10. I have no idea what was wrong, or what was changed to fix it.
And now another software update unfixes the problem: the computer can see both the printer and the scanner, but not communicate with them.
