Oh, I misunderstood. I thought you are trying to get workouts from Intervals → Wahoo. But it’s the other way around. They (Wahoo) recently changed their API access. I bet they no longer give access to Systm Workouts to Third Party Applications 
I would open a ticket at Wahoo’s side to ask if they limited access.
This was about activities, but the timing seems to be the same, and so probably they don’t give any access to workouts anymore, too.