Nextcloud Log File

I was able to locate documentation that may be able to assist you with locating the log file. It looks like there's a setting in the "config/config.php" file, based on the documentation. I would also check the "messages" and/or "syslog" files for possible insight.

https://docs.nextcloud.com/server/14/admin_manual/configuration_server/logging_configuration.html

If this doesn't help, you can try using the "find" command to manually locate the log file. An example I would try is the following:

sudo find / -name nextcloud*