MS Project Error Message

I am receiving the following message occassionally when attempting to “save” a project file. I am 100% confident that the file name and path are correct and I have not changed the file format. Thoughts?
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The file cannot be opened.
Check that the file name and path are correct.
Check that the file format is recognized by Project.
Project files saved in a version ealier that Microsoft Project 98 can’t be opened.
If your file is from an earlier version, open it in that version, click Save As and save in MPX format. Open the MPX file in the current version of Project. When you use this method, project data is imported, but formatting is lost.
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