I quit the trial. I encountered too many problems such as this one and was not even able to get it to run even once.
Maybe if this issue gets feedback how to solve it, I might consider retrying this plugin.
In the constructor of CryptKey.php, I changed keyPermissionsCheck from true to false which seems to have solved the issue of wrong file permissions. However, the validation still won't return any results.
I found out that there has been an update to the OAuthServer that basically breaks it on Windows due to file permission checks that are incompatible to the permissions used on Linux.
In another project that has been discussed and fixed by disabling the checks. Read more about that here: https://github.com/laravel/passport/pull/454
After researching this topic, I found the solution. You need to install the UrlRewrite Module for IIS from Microsoft and then you can either configure it using the IIS Manager or manually by adding the following lines to the web.config file: