Internet Explorer - Constructor not found

While my application works in Chrome & Edge, it gives a 'constructor not found' for every widget on the page, for all users.

The error is unfortunately not very descriptive:

{description: "evalModuleT...", info: Object {...}, message: "evalModuleT...", name: "Error", src: "dojoLoader"}

Loading module widgets/widgets failed! Check script file for syntax errors.
No constructor found for widget CheckboxSelector.widget.checkboxselectormf undefined
No constructor found for widget HTMLSnippet.widget.HTMLSnippet undefined
No constructor found for widget Rota.widget.Rota undefined

Anyone knows what might be causing this on Internet Explorer?