When the update to 11.5.1 appeared on the net (xlr8yourmac.com and macintouch,com, etc.) I tried to get the update via the System Preferences Software Update pane. That caused an "Software Update Preference Error" to pop up every time I tried it. After trying Silentknight (did not work, said it finished the download but failed to produce the update on a restart) I was finally able to get updated to 11.5.1 by accessing the App Store and downloading the full 11.5.1 installer. Evidently Apple has changed the update process. A little search produced this page (<https://mrmacintosh.com/macos-big-sur-full-installer-database-download-directly-from-apple/>), which describes the new process. After installing the full 12GB Big Sur 11.5.1 system installer, those "Software Update Preference Errors" disappeared.
I suppose I should send a note to the developer of Silentknight about this too.
I suppose I should send a note to the developer of Silentknight about this too.