Check the TNG file path in the address bar / URL
It must point to the standard TNG path.
Below is a example (it will differ per installation):
http://127.0.0.1:7080/registertng?6C294 ... 034731D124
We had an instance that after 'http://127.0.0.1:7080/' it had 'admin/'.
After removing 'admin/', the register.tng file was downloaded.