The SharePoint connector allows you to use Service Principal for downloading and uploading files, but it requires the Sites.All permission, which gives the user access to all the Sites in the tenant. This article describes how to set it up.
Connector tools that use "Sign in via Internet Browser" do not work correctly with Designer or Server 2021.3.2. To use those tools, you should upgrade to version 2021.3.3 or later and then download and install the tool version that says " Fixed blank screen issue when authenticating in Designer 2021.3.2.54175. Please note that you also need to update to Designer 220.127.116.11061 or later.." Alternatively in the short term, you can use version 2021.3.1 of Designer or Server.