The message says it all. I should have noticed that a long while ago. When saving a file as -- that is, when starting a work session by loading a graph from a file, I use to save the new graph as using a different name.
It turns out that the file name displayed on the window bar does not update and still shows the previous file name.
Quite confusing.
Thank you very much for your report.
This bug is now fixed in the current svn code line (commit #10218).