CAD tip # 10557:

Another user is updating the project and your changes cannot currently be saved.

This may be caused by another user currently changing the project settings (Project Setup). In this case wait for completing his/her changes.

Or there are obsolete data stored in Plant project cache. In this case perform purging of the local project cache:

• Exit AutoCAD Plant 3D
• Run the application PnPLocalDataCachePurger.exe, which is located in the Plant 3D installation folder. The default path for AutoCAD Plant 3D 2016 is:

  C:\Program Files\Autodesk\AutoCAD 2016\PLNT3D\PnPLocalDataCachePurger.exe