submitting with another browser is no solution. I have the same trouble and there is no way to solve. even if I try to restart the exercise, or to get code, I get "could not restore workspace".
I've checked my code many times. No way. Think it's a bug. The worst is that if you don't solve it you cannot continue with the following exercises and complete the course, and this is very frustrating, especially if you paid for a pro-subscription.