If you tried to use an older version of Scratch to open a Scratch file made by a newer version of Scratch, you might get that error. If that is the case, you might be able to just try using the newer version of Scratch to open it.
But all other scenarios that I can think of it do not have a happy ending. It usually means that the file is damaged, possibly because of interrupting a Save operation or some other reason. I've never heard of anybody being able to recover from a damaged file.
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