The most common error… Can you verify the file ownerships in the InvoiceNinja directory match your user account for the web server software? For Ubuntu, whether you use NGINX or Apache, it ought to be www-data:www-data owning every file.
And if you have any dumps in storage/logs/ that is most often the best ways to find where the errors are.
And do you host any other PHP web applications on your server also? Such as a “Grav” blog, or other webapp? Conflicting versions of PHP can cause issues, that can be rectified by being more explicit with PHP references in the configuration of the NGINX file.