Have been frustrated by still not being able to scan using my HP M29W device using UM 20.04. Reached out to VueScan software support (https://www.hamrick.com ) and got the following suggestion that has made scanning work again under most situations. (HPLIP/UM still seem to be broken, however, and I hope for a solution in the near future.)
- sudo apt purge ippusbxd
- then check that libcanberra-gtk-module and libcanberra-gtk3-module are installed
Having done these things, vuescan can be run successfully. Sometimes when multiple usb connected scanners are present there are troubles, but most times scans work.
Perhaps this will help others until a real fix is available.