I saw I thread in here with almost the same problem. I share my Gant with some group members on cloud and after one of them saved on hers computer I cannot. it says Got HTTP 409 error when I save. why?
HTTP 409 means that there were conflicting updates, and this seems to be the case indeed, as you described. Should it happen when working with existing projects, GanttProject is supposed to show a friendly dialog with a few options. However, it seems that when both you and your colleague create a new project with the same file name.
We’ll add code to handle this in GanttProject (issue in the tracker). Meanwhile, you can save the project with some other name.