More info, hopefully it's helpful.
Recent error log entries look like
2020/10/03 16:06:18 [error] 352#352: *1108 FastCGI sent in stderr: "PHP message: PHP Warning: DOMDocument not found, unable to use sanitizer in /app/dokuwiki/vendor/simplepie/simplepie/library/SimplePie/Sanitize.php on line 271PHP message: PHP Warning: DOMDocument not found, unable to use sanitizer in /app/dokuwiki/vendor/simplepie/simplepie/library/SimplePie/Sanitize.php on line 271" while reading upstream, client: 172.17.0.3, server: _, request: "GET /doku.php?id=wiki:syntax HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "wiki.myserver.org:443", referrer: "https://wiki.myserver.org/"
Recent debug log entries look like:
12:29:35 172.17.0.3: require_once() is deprecated. It was called from require() in /app/dokuwiki/inc/load.php:147 Autoloading. Do not require() files yourself. should be used instead!
12:29:35 172.17.0.3: INFO_DEPRECATION_LOG:AFTER event triggered before event system was initialized