devel-su
mask the following: (just in case any of these triggers a reboot in the middle of OTA update)
systemctl-user mask voicecall-ui-prestart.service
systemctl-user mask mission-control-5.service
systemctl-user mask booster-silica-media@.service
systemctl-user mask voicecall-manager.service
reboot
devel-su
if installed, unapply and remove all patches
if installed, uninstall patchmanager
if used, ssu dr mousetracker
if used, ssu dr chum
if used, ssu dr sailfishos-chum
ssu release 4.5.0.21
double check whether these three repos are pointing to the 4.5.0.21 version:
ssu lr
  adaptation-common should be: https://releases.jolla.com/releases/4.5.0.21/jolla-hw/adaptation-common/armv7hl/
  adaptation-community-common should be: https://repo.merproject.org/obs/nemo:/testing:/hw:/common/sailfishos_4.5.0.21/
  adaptation-community-treltexx should be: https://repo.merproject.org/obs/nemo:/testing:/hw:/samsung:/treltexx/sailfishos_4.5.0.21/
ssu ur
zypper ref
zypper dup --details
unmask the following:
systemctl-user unmask voicecall-ui-prestart.service
systemctl-user unmask mission-control-5.service
systemctl-user unmask booster-silica-media@.service
systemctl-user unmask voicecall-manager.service
reboot