@KatiLT This issue was resolved some time ago. You should currently have no issue launching breakouts within GoToTraining. Are you able to reproduce the behavior on newly scheduled trainings?
This happened in a training event we had yesterday (scheduled last week, held yesterday). We started the activity, selected the folks into the correct breakout groups, and then the "start" button didn't work at all. It was a light pink, and not active, so we weren't able to use breakout groups.