the installed version of fwupd is 1.5.7-4. the latest available version of fwupd is 1.5.7-4pureos0.1. “aptitude upgrade” says:
Resolving dependencies... fwupd-amd64-signed depends upon fwupd (= 1.5.7-4) -> Installing fwupd 1.5.7-4 (now) The following packages will NOT be UPGRADED: fwupd{a} (R: secureboot-db, S: gir1.2-fwupd-2.0, R: fwupdate, R: gir1.2-dfu-1.0, R: libdfu-dev, R: libdfu1) No packages will be installed, upgraded, or removed.
i believe this failure is happening because fwupd-amd64-signed requires an exact version of fwupd:
Depends: fwupd (= 1.5.7-4)
hope this report helps get this unblocked.