It appears the issue is tied in with the latest UiPath.System.Activities update 21.4.0.
When on updates up to 20.10.4, the Build Data Table activity works as intended - You can create a new activity, open the builder for the datatable, and are able to save changes without error.
Immediately upon upgrading to 21.4.0, you can create a Build Data Table activity, but when trying to simply open/close the builder you get the following error:
Downgrading to 20.10.4 resolves the issue, however this is not ideal as 21.4.0 brings several improvements that are desirable. This needs fixed ASAP.