"František is happy with his new girlfriend."
Translation:Se svou novou přítelkyní je František šťastný.
"František je šťastný se svou novou přítelkyní" is one of the accepted translations. If that was your actual answer, this may be a system error.
There is an intermittent grading bug affecting multiple courses. Duolingo staff are working to identify the cause and find a solution. Contributor teams cannot do anything to resolve the problem directly, apart from confirming that a rejected translation should be accepted.