Hi,
I have run the wordpress and plugin updates.
I really should remember WP_DEBUG <grin>. With that on the wp-admin pages come back with code 200 rather than 500 but completely blank. Server logs show
"GET /wp-admin/ HTTP/1.1" 200 366
so that's 366 bytes. Still no notices in the error log. Inserting my own error_log() call into, say, wp-config.php shows that error logging is working.
PHP version 5.6.22.