How does nBill output on the frontend, as a component or a module?
Component.
Please clarify what you mean when you say they cannot login...
Enter credentials, click login. Same login window appears stating one needs to login to use this resource. Infinite loop.
are there errors present in the browser console?
No errors.
Have you contacted nBill to see what the conflict is?
No, as nBills programmig hasn't changed in some time and the break occured only after updating to Gantry5.
www.nbill.co.uk/