It might mean the file is corupted. This can happen when you copy the file from another computer, or you did the simualtion and then switch back to modify it etc.
If you can still open the file, perhapes you can copy and paste the structure in FDTD and use the automatic conversion to script file. Then you can modify it with script to create EME file. https://optics.ansys.com/hc/en-us/articles/1500007185181-FDTD-Project-to-Script-Prototype
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