Since I posted the above, we’ve upgraded to WordPress 4.6. Also tried to upgrade BuddyPress again with the same result.
We’ve run this with the debug settings activated in the wp-config.php file but have received no error messages. Nor either in the wp-content/debug.php file, nor in the server’s php_log.
Thinking incompatibility issue with other obsolete plugins…testing now…will report…
@djpaul – We upgraded from WP 4.2 and whatever the BuddyPress version was prior to 2.3.3 (sorry I don’t recall, but we’ve been staying up-to-date, upgrading as new versions become available.)
@danbp – Not quite sure I follow your question, but…we upgraded from the Network Admin interface, not the individual site, and then performed a Network upgrade.
For whatever it’s worth, we do not see this error in any of today’s httpd error logs…