Found the problem: it was due to the missing semicolon at the preceding directive. I am not sure why I didn't see this before (and I was looking) but after a couple of restarts and trial-and-error attempts, I spotted the following warning:

[warn] ... only the last index in "index" directive should be absolute in /etc/nginx/sites-enabled/mydomain.com.conf:9

The directive preceding access_log was index and it lacked a semicolon. So, stupid me, of course but no brownie points to nginx for being so permissive with the syntax.