Today when I log into my web site and go to view an Joomla article that is set to use SPPB Editor, when I click the blue Preview button, the preview popup window appears, and I see the message:

The most recent request was denied because it had an invalid security token. Please refresh the page and try again.

This happens on all my SPPB pages in the Joomla Article Manager -- doesn't matter if they were created a long time ago, or just this past week.

This happens in all web browsers, even after cache and cookies deleted.

Any ideas why this would occur? Joomla! 3.9.24 / PHP 7.3 (also occurs with PHP 7.4)