Unexpected File Format Error

Is anyone else experiencing this error message.
We have experienced this in SW2004 but have seen a large increase in occurrences in SW2005 SP0.1 This error appears when opening a previously
worked on file and to date have no solution apart from re-loading from back-up and therefore loosing all work. This error sometimes shows up by receiving a "Unable to save" dialog, then when trying to open said file it gives a "Unexpected file format" error. Can anyone suggest a cause or better still a solution.
I will be posing this to my VAR, so I will inform you of there response.
Simon Miller Cube3
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