Recently an AutoCAD Plant 3D user experienced a crash that left the project locked for himself and all other users.
When he next attempted to save changes to a drawing in the project, the following message appeared:
Project Currently in Use
Another user is updating the project and your changes cannot currently be saved.
Please try again later. If you encounter this message repeatedly, please contact
Unable to Perform Operation
The project must be closed by all other users in order to perform this operation.
The project is currently open by another user. Please try again later.
Similar to how AutoCAD locks a file with a .dwl file when a drawing is being edited, AutoCAD Plant 3D and AutoCAD P&ID will lock the project file with a project.xml.lock and will also create these four other file locks in the project folder where the project.xml file is located:
Once these five files are deleted after Plant 3D or P&ID are closed, then the project can be opened and edited with no further error messages.
Special thanks to Chuck Jolin for the problem description
and confirmed solution.