Which USB cable works for Kontrol S61 MK3?

Thank you very much for that report ! I think that it is necessary to have illustrated exactly what kind of problems that users are up against with respect to cables ! From what I have been able to gather so far then apparently maybe passive Thunderbolt 4 quality cables are the answer to your question. (the passive is important) But not having neither a S-Series keyboard to test with nor a computer with an USB-C port either for that matter then all I can do is to suggest that as to put to the test ! (test since it has been reported to work though not widely tested as a fix since mostly there appear to have been more possibly overlapping problems for users)

And notice ! Apparently then also passive Thunderbolt 4 quality cables are relatively costly/expensive !

With respect to the S-Series MK3 then there appear to be no (other) clearly defined specifications that people can use and look for when buying cables and then be sure that the cable works and not even the cable supplied with the keyboard works for everyone, which IMO ought to be considered as a huge problem since it adds greatly to confusion and muddles trouble shooting !

I am not suggesting the use of a passive Thunderbolt 4 cable as the only solution , I were merely suggesting it as a solution ! (when a user writes stuff like : "I found only one cable that worked, purchased 6 from referrals on this site"). In fact , me not having anything to test with (my computer is too old to have an USB-C port and I do not have a S-Series MK3 either) then I can not even be sure that my suggestion is a solution , so I suggested it as to be tested ! (Quote my post above : "all I can do is to suggest that as to put to the test !")

To make matters a lot worse then Matthew_NI, who in my opinion is the only qualified interface to any true insight here, is bound by confidentiality agreements and NDAs (Non Disclosure Agreements) so he is not even at liberty to add his to any true discussion on the matter. I mean , even if Mathew came and gave his 'explanation' then I would most likely think that there still were some room for some type of discussions on the matters at hand. But when you can not even get the truth as seen by NI then I don't think that there is any real grounds for any discussions as to why and what.

However , I will add this ! : No matter how you or even how Matthew_NI comprehend me or how little credit that some users here tends to give me here then my thinking on things actually always at least tends to seek to be rather profound and I do try to see things in a greater perspective. And that includes matters like e.g. cables and electronics. As written more places in this forum then I do not claim to have the proper formal education to do any in depth analysis but what I do have is at least some (compiled) understanding of among other electrics and electronics and a good deal of common sense to put that into good use !