When opening a file in Solo, Archicad creates an ".lck" file to prevent other users / applications modifying the very same project as you currently do. Check if any ".lck" file exists with the same name as the affected project. If you find any, close all running Archicad instances and delete the file with .lck extension exists next to the project - which could not be shared.
Incorrect permission settings applied for the Graphisoft folder and its subfolders
Make sure that the affected user has full access to the following folder and its subfolders: