Hi Arezoo,
Sorry about the trouble! Are you still receiving this error? Typically this error will show up if the file in your UNDO folder has become corrupted. You can check a few things:
1) Make sure that you have full read/write access to the Documents/Zemax/UNDO folder.
2) If you have any cloud services, like Microsoft OneDrive, make sure that the files are being stored locally and not on a network location.
3) Trying saving the file and restarting OpticStudio.
Please let us know if you continue to see this error.
Ethan
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