Cannot add items to invoice in any browser

When populating the invoice content type I click the "add" button to add an item with pricing to the invoice, but the button does nothing.

I have tried to save the content type with the fields populated but not added to the invoice and it says that I need to add items to the invoice.

I have had successful (older) installations of invoice working on other sites.

I have also uninstalled and removed invoice from the site and reinstalled it to see if there was some issue with the first install, same issue this time.

Thanks in advance!

Ash