Huzzah! I apologize to everyone since it was my router settings that cause the issue in the first place.
There were global problems caused by the Qobuz migration, not just on your end. I tried to link up my account Qobuz for native integration with Volumio literally every day this month, and always got the same “Bad Request” error page. Then, about two days ago, I got a different error message with a much longer name (I didn’t bother to write it down, but it was something to the effect of “Could not retrieve token” but with a bunch more words). I was going to post about it here, but I didn’t want to get all your hopes up. Then, this morning, I tried linking it again, and suddenly it worked. 
Perhaps but I was told repeatedly that the problem was on my end and that it was my router - so I went out and purchased a new, expensive one (different model) then spent days setting everything up. I still had the issue so I returned it for a different brand and went through setup. this took a couple of weeks due to work travel but eventually that 3rd router resolved all the Qobuz issues for everyone.
Thanks for your patience .