Cannot add datasources in on-premises gateway

Previously I have been able to add, delete, and re-create datasources for the gateway via the 'Manage' Gateways' menu in the Power BI Service, but no longer.

When trying to add a new datasource, I get the following error message;

I have installed the latest gateway version, which seems to be working properly

Looking at the gateway logs, there are messages about TLS and the framework version. I have uninstalled the .NET framework, and reinstalled version 4.0, 4.5, and 4.6 in succession, but the error messages remain.