Apophysis

I have been using Apo for quite some time now. When I opened the program today and began to work in it, I began having a problem. When hitting the undo button, my current flame I am working on reverts to a flame that I had saved previously, on a different day. I checked the program folder and located an 'apophysis.undo' file, and opened it with notepad. This showed an undo history of approximately 3 flames. The same flames that I get today, when clicking the undo button. I have tried to delete the history and save the file, but I get an error that I cannot save the file.

Can anyone suggest a remedy, or tell me how to reset the undo file?

Thanks in advance :)