Tried the switch to PHP 7.1 just now. Jetpack monitor reported the site went down not much later. (It hadn’t.)
However, the logs did show:
xx.xx.xx.xx – – [14/Jan/2017:10:47:44 +0100] “HEAD / HTTP/1.1” 500 – “-” “jetmon/1.0 (Jetpack Site Uptime Monitor by WordPress.com)”
Reproduced this behavior using curl -I. GET requests, however, were working fine–which is why I hadn’t noticed anything off:
xx.xx.xx.xx – – [14/Jan/2017:10:51:05 +0100] “GET / HTTP/1.1” 200 14960 “-” “Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36 Edge/14.14393”
Anyway, the issue disappeared as soon as I disabled caching. The only way, though, to have it stay gone with caching on, was to go back to PHP 7.0.
(Unfortunately, I don’t have a more detailed (PHP) error log at the moment. Saw the issue both on mod_rewrite caching as on PHP caching, so I’m assuming it’s PHP at work.)