Running Update Manager resulted in an error message to the effect that a Personal Package Archive (PPA) for the Opera web browser, was missing and/or unavailable.
Now, since I switched from Opera to Vivaldi some time ago and uninstalled Opera, this message doesn’t really apply to my current system and is, therefore, simply an annoyance. However, eliminating any future occurrences of the error is simply a matter of disabling the relevant PPA in the list of repositories.
The offending entry was found under Update Manager – Edit – Software sources – Additional repositories. Unchecking the box next to this entry solved the “problem”.