Theme Sniffer License Warnings

I’ve searched the forum, web, developer handbook etc. but couldn’t find a solution for my problem.

When I run the Theme Sniffer Plugin on my WP theme, I get the following two warnings for the readme.txt file:
WARNING No matching license critera could be determined from: GPLv2 or later!
WARNING Unable to determine License URI with an invalid License supplied in readme.txt!

In the corresponding fields License: and License URI: in the readme.txt header, I’ve tried so far:
1.) Leave both blank
License URI:

2.) Put a local copy of a license in a subfolder:
License: GPL/GPLv2 or later/…
License URI: ./license/license.txt (both with and without leading dot)

3.)The standard themes (twenty17/19/20) entries, which also generate the warnings:
License: GPLv2 or later
License URI:

Has anyone an idea of how to get rid of these warnings?
Would the theme be rejected if I let the license fields blank or follow no. 3.)?

Any help appreciated

Maybe some examples of valid license/license URI strings? What do Envato experienced authors put in there?