Resistance in Zwift Academy

I experienced something strange during Zwift Academy training #3 (the long one). We have to ride twice for 8 minutes at 215W. At some point, it was as if I were in a very steep hill, something like 25%, even on 1st gear almost impossible to pedal. The situation stayed the same for the rest of the training until I decided to quit. Tried the training 3 times this week, same resistance problem. I switched to another ride and everything was ok. I’m gonna try the short version.