Internal Server Error I can't interpret

Nextcloud version (15.0.4):
Operating system and version (Ubuntu 18.10):
Apache or nginx version (nginx 1.15.5):
PHP version (eg, 7.2):

The issue you are facing: I get an internal server error pointing me to the server log, but I’m green enough to not know exactly where to find it. I can find access.log and error.log; is it one of these? Thanks, everyone, for taking pity on a poor noob!

Is this the first time you’ve seen this error? (N):

Steps to replicate it:

  1. Point my browser to localhost/nextcloud.index.php

The error.log sounds right, if it is configured by nginx. Is there content in the error.log? If not, increase the log level of the web server.

Okay, here’s the content from the error.log:

PHP message: {“reqId”:“7UuoqE0c36OK601i22FC”,“level”:3,“time”:“2019-02-28T02:38:25+00:00”,“remoteAddr”:“”,“user”:"–",“app”:“PHP”,“method”:“GET”,“url”:"/nextcloud/index.php",“message”:“chmod(): Permission denied at /var/www/html/nextcloud/lib/private/Log/File.php#141”,“userAgent”:“Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:65.0) Gecko/20100101 Firefox/65.0”,“version”:""}" while reading upstream, client:, server: _, request: “GET /nextcloud/index.php HTTP/1.1”, upstream: “fastcgi://unix:/var/run/php/php7.2-fpm.sock:”, host: “”, referrer: “

I’m confused by the backslash-forwardslash separators in the pathnames. Is there a reason they’re not just /? Is that a PHP thing?