Ok I just ran some more tests on a different project, same issue. But it only happens when saving on the shared folder so it might be a issue with my version of Virtual Box (I had issues of this kind with mmap function, I guess you might have changed the way you save it perhaps).
I noticed another writting issue when trying to save a new project from Python. If I create a new psx project I get this error.
>>> doc.save('project.psx')
False
2015-12-18 09:53:31 Saving project...
2015-12-18 09:53:31 Finished processing in 0.000361 sec (exit code 0)
2015-12-18 09:53:31 Error: Can't save project
No project file is written and it only happens if I save it on the VM. Works ok if I save it from the GUI or to the windows shared folder.
I also had this error but can't reproduce it.
2015-12-17 15:48:33 Saving project...
2015-12-17 15:48:35 saved project in 0.835446 sec
2015-12-17 15:48:35 Finished processing in 0.835703 sec (exit code 1)
2015-12-17 15:48:35 Loading project...
2015-12-17 15:48:35 Finished processing in 0.0776 sec (exit code 0)
2015-12-17 15:48:35 Error: Disk IO error