Not meaning to threadjack, but I have the exact same issue with my 77" S93C, and it is extremely frustrating since Samsung has locked large portions of the functionality against the Samsung Account (general TV usage, updating firmware, installing Apps, using the voice functionality on the remote etc.)
I've repeatedly (i.e, more than 20 times) tried associating the Samsung Account with the TV; resetting SmartHub, rebooted the TV (also numerous times), as well as running diagnostics in order to check IP/DNS and callback functionality against Samsung's servers – all to no avail.
The issue behaves exactly the same as the OP reports, the account is (seemingly) logged in to the TV (stating "You are logged in to your Samsung Account"), then it asks for a six-digit(!!) PIN-code, and upon entering it, the same procedure starts again.
Thinking it was a software issue, the TV was reset and the firmware attempted to be updated, but since it *also fails* when asking one to register or log in to a Samsung Account (which doesn't work) in order to update, this is not an option.
Extremely disappointing for such a TV, and it comes on top of the cumbersome remote (which is non-backlit, if you can believe it) and not good at inputting PIN codes or passwords.
Even with a USB keyboard attached one simply cannot just type this in, as it also behaves as a remote.
Assistance kindly requested.
