Split not working on User Story

[Glenn-Gidlof]

Trying to use the Split function is not working on User Story. Tested this on a different collection and it works. The failing collection does have modifications to the User Story WIT. Custom fields, additional tab, hidden fields.
The console output doesn't show anything after clicking the Split button in the dialog.
Running v0.1.6 of the Split extension.
TFS 2018.2 OnPrem

I suspect this is happening because of one of our custom fields but don't know which one or how to identify it, or if there is a way to fix it.