The following is another great guest post from Product Support Specialist Lance Coffey.
We are starting to see a number of users reporting "Data in file is corrupt" error messages after upgrading a project to Revit LT 2014.
After some investigation, we have found that this can happen when saving an upgraded file on top of a previous version. The reason is that some of the prior version elements remain unchanged in the file, which then puts the project in a state which cannot be used by Revit LT, and when reopening the project, Revit LT will report data in the file is corrupt.
If this happens, recover a backup copy of the project before it was upgraded.
To avoid the issue, when upgrading a project in Revit LT 2014, use Save As to save to a new filename.
Once upgraded and saved to a new name, you can save the 2014 project normally.
Our developers are aware of this, and are currently working to address the issue.