UPDATE: I have found a workaround to my problem. Rather than upgrading to the latest version of Adobe Flash Player through System Preferences, I first uninstalled the older version by following the instructions of the following link.
Uninstall Flash Player | Mac OS
I then downloaded and installed the latest version of Adobe Flash Player from the following link.
Therefore, it appears that the problem lies with the update program and not my system configuration. It would be good to find out what causes this problem so as to avoid/resolve similar issues in the future....