Yeah Autodesk. Great job in not fully testing your software before releasing it. Hopefully only 200 people are using Revit LT anyhow.
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 <http://wikihelp.autodesk.com/Revit_LT/enu/2014/Help/0082-Revit_LT82/0119-Start_a_119/0171-Saving_F171/0175-Backup_a175> 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.