Searching for server log

I set up Nextcloud 18.04 onto my Ubuntu 20.04 LTS server.
Nextcloud is starting, I create an admin account, I installed some apps. After a short while I had to reboot my Windows computer.
When I return to Nextcloud’s webinterface, I see an error which tells me to check the server log. I found 3 files named nextcloud.log; which are either empty or didn’t contain the given request ID.

I think I am looking for the wrong file. Where can I find the right file?

Best regards,

The log file is by default created in the Nextcloud data directory, except you’ve configured a different location.