For what it's worth it's STILL broken on 21.04, but I fixed by grabbing
hplip from HP's website and installing via the provided shell script instead
of the broken version shipped with Ubuntu.

-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-
  Eskimo North Linux Friendly Internet Access, Shell Accounts, and Hosting.
    Knowledgeable human assistance, not telephone trees or script readers.
  See our web site: http://www.eskimo.com/ (206) 812-0051 or (800) 246-6874.

On Wed, 5 Jan 2022, Michel-Ekimia wrote:

> Date: Wed, 05 Jan 2022 16:02:22 -0000
> From: Michel-Ekimia <email address hidden>
> To: <email address hidden>
> Subject: [Bug 1871011] Re: USB scanning broken on focal by ippusbxd (was:
>     hpmud and ippusbxd conflict)
> 
> We had several clients with HP printers that faced this problem.
>
> This should really be fixed on LTS 20.04 as there is no simple
> workaround for the user
>
> If ippusbxd is non functionnal I guess it should be removed on next
> 20.04.4
>
> ** Summary changed:
>
> - USB scanning broken on focal by ippusbxd (was: hpmud and ippusbxd conflict)
> + USB scanning and printing broken on focal by ippusbxd (was: hpmud and ippusbxd conflict)
>
> ** Description changed:
>
> + SOLUTION : sudo apt remove ippusbxd
> +
> + ---- Initial report -----
> +
>  On Ubuntu 20.04 focal (with -proposed enabled). HPLIP=3.20.3.
>
>  Previously there was a libmtp (udev) bug erroneously detecting HP
>  printers as mtp devices. This was fixed. Now, I can try to scan-- from
>  both simple-scan and hp-scan, hplip errors out:
> -
>
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 427: Found interface conf=0, iface=1, altset=0, index=1
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 389: Active kernel driver on interface=1 ret=0
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 515: invalid claim_interface 7/1/2: Device or resource busy
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 427: Found interface conf=0, iface=0, altset=1, index=3
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 389: Active kernel driver on interface=0 ret=0
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 515: invalid claim_interface 7/1/4: Device or resource busy
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 427: Found interface conf=0, iface=3, altset=0, index=9
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 389: Active kernel driver on interface=3 ret=0
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 515: invalid claim_interface ff/4/1: Device or resource busy
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 427: Found interface conf=0, iface=0, altset=0, index=11
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 389: Active kernel driver on interface=0 ret=0
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 515: invalid claim_interface ff/cc/0: Device or resource busy
>
>  Printer/scanner is an Officejet 4635. Scanning previously worked just
>  fine a few months ago. Regression?
>
> -- 
> You received this bug notification because you are subscribed to a
> duplicate bug report (1873748).
> https://bugs.launchpad.net/bugs/1871011
>
> Title:
>  USB scanning and printing broken on focal by ippusbxd (was: hpmud and
>  ippusbxd conflict)
>
> Status in HPLIP:
>  Invalid
> Status in hplip package in Ubuntu:
>  Expired
> Status in ippusbxd package in Ubuntu:
>  Confirmed
>
> Bug description:
>  SOLUTION : sudo apt remove ippusbxd
>
>  ---- Initial report -----
>
>  On Ubuntu 20.04 focal (with -proposed enabled). HPLIP=3.20.3.
>
>  Previously there was a libmtp (udev) bug erroneously detecting HP
>  printers as mtp devices. This was fixed. Now, I can try to scan-- from
>  both simple-scan and hp-scan, hplip errors out:
>
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 427: Found interface conf=0, iface=1, altset=0, index=1
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 389: Active kernel driver on interface=1 ret=0
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 515: invalid claim_interface 7/1/2: Device or resource busy
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 427: Found interface conf=0, iface=0, altset=1, index=3
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 389: Active kernel driver on interface=0 ret=0
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 515: invalid claim_interface 7/1/4: Device or resource busy
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 427: Found interface conf=0, iface=3, altset=0, index=9
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 389: Active kernel driver on interface=3 ret=0
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 515: invalid claim_interface ff/4/1: Device or resource busy
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 427: Found interface conf=0, iface=0, altset=0, index=11
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 389: Active kernel driver on interface=0 ret=0
>  Apr 05 21:27:37 dani-mbp15 simple-scan[3464]: io/hpmud/musb.c 515: invalid claim_interface ff/cc/0: Device or resource busy
>
>  Printer/scanner is an Officejet 4635. Scanning previously worked just
>  fine a few months ago. Regression?
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/hplip/+bug/1871011/+subscriptions
>