Steve
In v4.11 there is a defect that has already been fixed for the 4.12 Patch that is being readied for release. The issue is caused by (this is when I have seen it happen anyway) when you have been editing a project and get called away from your work for a while and come back to it some time later and hit save - some temp file is getting cleaned up and because it is absent it stops you from saving the VCE file. In most cases I have been able to Save As and save it with a new name and continue to work - however I did have one time when I could not do that and had to VCL out my data and VCL In the data to a new project (that was painful for sure but it has only happened once).
As I say this issue is fixed in the next release which I hope will be out shortly.
Alan