Most likely it is due to conflict of the parameters and the simulation file cannot be created. Please check the the ranges of the optimized parameters.
Other minor remedies are to reboot the computer; copy and paste the objects except the varFDTD to a new project file.
If you are sure the parameters have no conflicts and the minor remedies do not work, please send us an email through you local rep or our support email address if you know, and mention this post. Then I can check it for you.