UnderDogMiller
Make sure the files aren’t corrupted somehow, make sure your webserver is not blocking the URL or mangling it, check your server logs to see if anything is being logged, etc. I would also make sure your registered app in azure has the correct url configured.
You can also try downloading the raw plugin files from GitHub, uploading the raw auth-oauth2 folder to your plugin folder, moving the phar plugin outside the plugin directory, going to your database.ost_plugin table, set isphar to 0 for that plugin, and retest.
Cheers.