Installation page is still blank, how to debug this

Hi, after unzipping the files into a directory I go to the index.php in that directory, but nothing loads; it stays a blank page.

How should I debug this? Is there any log I can check to see what goes wrong?

I have:
Apache2- server (LAMP) on Ubuntu 20.04
php 7.4.3
and all modules mentionned in the Requirements page installed.
Thanks!

Here you can find how to enable debug:

https://akaunting.com/docs/developer-manual/debug

Thank you. The debug reports: The stream or file "/var/www/[mydomain]/storage/logs/laravel-2020-09-14.log" could not be opened in append mode: failed to open stream: Permission denied

In this directory there is no log created (maybe because of the permissions problem?), but I don't understand: all the directories have chmod 775 and all the files within the application seem to have permissions 664. That should work.

I found this thread:
https://akaunting.com/forum/discussion/installation-update/permission-denied-for-laravellog
but that doesn't propose a solution.

Meanwhile I installed Akaunting also on a hosting service (via Softaculous). This worked, but only when I choose to install via http, not via https. After installation via http, https can be used (adjusting htaccess).

Maybe this causes also on my vps the problem: I have already pre-runned certbot and a working certificate for the domain name linked to this vps, before the installation of Akaunting. I have not yet tested if this is the problem, because now I continue, for the moment, with the Softaculous-hosting-installation. Maybe I'll test this later, or earlier, if someone asks me to do via this forum.