Against the odds an almost identical install of EOS on a different laptop connected to the same printer works just fine. The plugin is installed correclty on both, I think.
journalctl -xe -f tells (sorry, the markdown code icon seems to have disappeared):
Sep 24 17:03:27 dell hplip.desktop[9383]: Downloading plug-in from:
Sep 24 17:03:27 dell hplip.desktop[9383]: warning: endeavouros distro is not found in AUTH_TYPES
Sep 24 17:03:27 dell hp-plugin[9383]: hp-plugin[9383]: warning: endeavouros distro is not found in AUTH_TYPES
Sep 24 17:03:35 dell hplip.desktop[9383]: error: Plugin download failed with error code = 8
Sep 24 17:03:35 dell hp-plugin[9383]: [9383]: error: Plugin download failed with error code = 8
Sep 24 17:03:35 dell hplip.desktop[9383]: error: file does not match its checksum. File may have been corrupted or altered
Sep 24 17:03:35 dell hp-plugin[9383]: hp-plugin[9383]: error: file does not match its checksum. File may have been corrupted or altered
Sep 24 17:03:40 dell hplip.desktop[9383]: Done.