This issue was fixed in Cinema 4D 2023.1.3.
This occurs in most Windows installations of R26 and later which use the Webview2 (Edge) component instead of Webview/Trident/IE.
This issue is under investigation, but appears to be related to an update Microsoft issued to the WebView2 component Dec 8 causing the URL to not resolve properly.
Currently the only mitigation is to disable the Quickstart (Edit-Preferences-Communication-Quick Start Dialog Enabled at Startup). That solution is available to commercial subscribers, though trial, student and other license types do not offer the option to hide the Quick Start.