There is a new version of the UK Data Loader now available which addresses this problem. The latest version (v4.1.2) can be found here.
We are aware of an issue when attempting to load OS MasterMap Topography data using the UK Data Loader after users have upgraded to ArcGIS Pro 2.4.
The issue is due to a configuration issue with the Data Interoperabilty Extension and we are working to resolve this as soon as possible.
In the meantime if you are planning on loading or updating MasterMap Topography data using UK Data Loader please hold off updating from v2.3 to v2.4. If you have already upgraded then you will need to reinstall v2.3 in order to use the tool.
The OS Highways, AddressBase and VML loaders are unaffected by this issue.