critical error

Hi,
After setting up the plugin, after setting the client and tenant ID, client secret and the URi, and then clicking on the save changes-button, a message appears saying:

Settings saved.
There has been a critical error on this website. Please check your site admin email inbox for instructions.

When I go back to the setup page, I notice Authorisation box is still saying:
You need to save settings with a valid Tenant ID, Client ID and Client Secret before you can proceed.

I dont’ get the plugin to work.

Running W2019 and IIS in Azure