Host couldn't share own screen

A weird thing happened in a practice webinar yesterday.  I was taking a newbie presenter through the usual steps and I couldn't share my own screen in the practice room. The green button was there, I am the account holder and host, but when trying to share, a message popped up saying that I was restricted from sharing my screen. After a check in the settings, everything was fine - screen sharing was enabled etc. Then, I did a quick test webinar with someone else and it worked fine as usual. This has never happened before and has unnerved me somewhat. Anyone else had this problem?